AWstats Term Definitions

Unique Visitor:
A unique visitor is a person or computer (host) that has made at least 1 hit on 1 page of your web site during the current period shown by the report. If this user makes several visits during this period, it is counted only once. Visitors are tracked by IP address, so if multiple users are accessing your site from the same IP (such as a home or office network), they will be counted as a single unique visitor.
The period shown by AWStats reports is by default the current month.
However if you use AWStats as a CGI you can click on the "year" link to have a report for all the year. In such a report, period is a full year, so Unique Visitors are number of hosts that have made at least 1 hit on 1 page of your web site during the year.

Number of visits made by all visitors.
Think "session" here, say a unique IP accesses a page, and then requests three other pages within an hour. All of the "pages" are included in the visit, therefore you should expect multiple pages per visit and multiple visits per unique visitor (assuming that some of the unique IPs are logged with more than an hour between requests)

The number of "pages" viewed by visitors. Pages are usually HTML, PHP or ASP files, not images or other files requested as a result of loading a "Page" (like js,css... files). Files listed in the NotPageList config parameter (and match an entry of OnlyFiles config parameter if used) are not counted as "Pages".
Any files requested from the server (including files that are "Pages") except those that match the SkipFiles config parameter.
The requests can be for anything... html pages, graphic images (including each slice of a table image), audio files, cgi scripts, etc... Each valid line in the server log is counted as a hit. This number represents the total number of requests that were made to the server during the specified report period. Because this is a mishmash of accesses, it's not very useful/accurate, except to boast about your traffic.

Total number of bytes for pages, images and files downloaded by web browsing.
Note 1: Of course, this number includes only traffic for web only (or mail only, or ftp only depending on value of LogType).
Note 2: This number does not include technical header data size used inside the HTTP or HTTPS protocol or by protocols at a lower level (TCP, IP...).
Because of two previous notes, this number is often lower than bandwith reported by your provider (your provider counts in most cases bandwitdh at a lower level and includes all IP and UDP traffic).

Entry Page:
First page viewed by a visitor during its visit.
Note: When a visit started at end of month to end at beginning of next month, you might have an Entry page for the month report and no Exit pages.
That's why Entry pages can be different than Exit pages.

Exit Page:
Last page viewed by a visitor during its visit.
Note: When a visit started at end of month to end at beginning of next month, you might have an Entry page for the month report and no Exit pages.
That's why Entry pages can be different than Exit pages.

Session Duration:
The time a visitor spent on your site for each visit.
Some Visits durations are 'unknown' because they can't always be calculated. This is the major reason for this:
- Visit was not finished when 'update' occured.
- Visit started the last hour (after 23:00) of the last day of a month (A technical reason prevents AWStats from calculating duration of such sessions).

A browser that is used primarily for copying locally an entire site. These include for example "teleport", "webcapture", "webcopier"...

Direct access / Bookmark:
This number represent the number of hits or ratio of hits when a visit to your site comes from a direct access. This means the first page of your web site was called:
- By typing your URL on the web browser address bar
- By clicking on your URL stored by a visitor inside its favorites
- By clicking on your URL found everywhere but not another internet web pages (a link in a document, an application, etc...)
- Clicking an URL of your site inside a mail is often counted here.

Add To Favourites:
This value, available in the "miscellanous chart", reports an estimated indicator that can be used to have an idea of the number of times a visitor has added your web site into its favourite bookmarks.
The technical rules for that is the following formula:
Number of Add to Favourites = round((x+y) / r)
x = Number of hits made by IE browsers for "/anydir/favicon.ico", with a referer field not defined, and with no 404 error code
y = Number of hits made by IE browsers for "/favicon.ico", with a referer field not defined, with or without 404 error code
r = Ratio of hits made by IE browsers compared to hits made by all browsers (r <= 1)

As you can see in formula, only IE is used to count reliable "add", the "Add to favourites" for other browsers are estimated using ratio of other browsers usage compared to ratio of IE usage. The reason is that only IE do a hit on favicon.ico nearly ONLY when a user add the page to its favourites. The other browsers make often hits on this file also for other reasons so we can't count one "hit" as one "add" since it might be a hit for another reason.
AWStats differentiate also hits with error and not to avoid counting multiple hits made recursively in upper path when favicon.ico file is not found in deeper directory of path.
Note that this number is just an indicator that is in most case higher than true value. The reason is that even IE browser sometimes make hit on favicon without an "Add to favourites" action by a user.
HTTP Status Codes:
HTTP status codes are returned by web servers to indicate the status of a request. Codes 200 and 304 are used to tell the browser the page can be viewed. 206 codes indicate partial downloading of content and is reported in the Downloads section. All other codes generates hits and traffic 'not seen' by the visitor. For example a return code 301 or 302 will tell the browser to ask another page. The browser will do another hit and should finaly receive the page with a return code 200 and 304. All codes that are 'unseen' traffic are isolated by AWStats in the HTTP Status report chart, enabled by the directives ShowHTTPErrorsStats. in config file. You can also change value for 'not error' hits (set by default to 200 and 304 with the ValidHTTPcodes directive. The following table outlines all status codes defined for the HTTP/1.1 draft specification outlined in IETF rfc 2068.
They are 3-digit codes where the first digit of this code identifies the class of the status code and the remaining 2 digits correspond to the specific condition within the response class. They are classified in 5 categories:
1xx class - Informational
Informational status codes are provisional responses from the web server... they give the client a heads-up on what the server is doing. Informational codes do not indicate an error condition. 
100 100 Continue
The continue status code tells the browser to continue sending a request to the server. 
101 101 Switching Protocols
The server sends this response when the client asks to switch from HTTP/1.0 to HTTP/1.1 

2xx class - Successful
This class of status code indicates that the client's request was received, understood, and successful. 
200 200 Successful
201 201 Created
202 202 Accepted
203 203 Non-Authorative Information
204 204 No Content
205 205 Reset Content
206 206 Partial Content
The partial content success code is issued when the server fulfills a partial GET request. This happens when the client is downloading a multi-part document or part of a larger file. 
3xx class - Redirection
This code tells the client that the browser should be redirected to another URL in order to complete the request. This is not an error condition. 
300 300 Multiple Choices
301 301 Moved Permanently
302 302 Moved Temporarily
303 303 See Other
304 304 Not Modified
305 305 Use Proxy
4xx class - Client Error
This status code indicates that the client has sent bad data or a malformed request to the server. Client errors are generally issued by the webserver when a client tries to gain access to a protected area using a bad username and password. 
400 400 Bad Request
401 401 Unauthorized
402 402 Payment Required
403 403 Forbidden
404 404 Not Found
405 400 Method Not Allowed
406 400 Not Acceptable
407 400 Proxy Authentication Required
408 400 Request Timeout
409 409 Conflict
410 410 Gone
411 411 Length Required
412 412 Precondition Failed
413 413 Request Entity Too Long
414 414 Request-URI Too Long
415 415 Unsupported Media Type
5xx class - Server Error
This status code indicates that the client's request couldn't be succesfully processed due to some internal error in the web server. These error codes may indicate something is seriously wrong with the web server. 
500 500 Internal Server Error
An internal server error has caused the server to abort your request. This is an error condition that may also indicate a misconfiguration with the web server. However, the most common reason for 500 server errors is when you try to execute a script that has syntax errors. 
501 501 Not Implemented
This code is generated by a webserver when the client requests a service that is not implemented on the server. Typically, not implemented codes are returned when a client attempts to POST data to a non-CGI (ie, the form action tag refers to a non-executable file). 
502 502 Bad Gateway
The server, when acting as a proxy, issues this response when it receives a bad response from an upstream or support server. 
503 503 Service Unavailable
The web server is too busy processing current requests to listen to a new client. This error represents a serious problem with the webserver (normally solved with a reboot). 
504 504 Gateway Timeout
Gateway timeouts are normally issued by proxy servers when an upstream or support server doesn't respond to a request in a timely fashion. 
505 505 HTTP Version Not Supported
The server issues this status code when a client tries to talk using an HTTP protocol that the server doesn't support or is configured to ignore.