There is a set of industry-agreed standard metrics that are associated with website audits and recognised by bodies such as jicwebs and ABC Electronic.  These measures serve to set standards by which advertisers and site owners can agree upon.

Page impression

"A file or a combination of files sent to a User as a result of that User's request being received by the server."

Effectively, a single request by a user should result in a single page impression being counted. Generally, when one clicks on a web link, the server will send a collection of files: style sheets and page furniture, such as images, as well as the html code that is the core content.  Counting these extra files results in a significantly over-inflated figure of viewed pages, which is misleading for both site managers and potential advertisers alike. Items which may need to be counted, such as advertising banners are counted as a separate metric.

Unique user

"A unique and valid identifier. Sites may use (i) IP + User-Agent, (ii) Cookie, and/or (iii) Registration ID."

The measurement of unique users in web traffic analysis is one of some difficulty. If an analysis uses a combination of IP address and User-Agent, the metric may have significant errors. Many people are assigned dynamic IP addresses by their ISP and, as a consequence, the number of IP addresses seen in access logs is inflated. Similarly, many people will use more than one type of browser. Conversely, many large organisations and ISPs direct web traffic through proxies that result in only a handful of IP addresses representing the browsing traffic of a much larger number of users.
Cookies can provide a more accurate estimate of users: however, many people choose to delete cookies periodically and these will be assigned a new cookie on re-visiting the site, thus inflating the user count.  Users may also choose to have cookies disabled in their browser and so are not countable by this method.
Counting individuals based on unique registration identifiers is perhaps the most accurate measure but is not appropriate as a metric for anonymous browsing.

Visit

"A series of one or more Page Impressions, served to one valid User, which ends when that User has not made a Page Impression for a 30-minute period."

People tend to view a short collection of pages on a website before moving off to another site. They may then return again later to read further content.  Visits are a measure of these bursts of activity. 

Visit duration

"The total time in seconds for all Visits of two or more Page Impressions, divided by the total number of Visits of two or more Page
Impressions."

A measure of the average duration of visits to a website. Each visit is given a duration by taking the difference in time between the first and last page impression in a visit. The results are summed and divided by the number of visits. Requests for single pages are excluded in this metric. 

Search

"The first Page Impression sent to a valid User as a result of that User's search request being received by the server."

When a user makes a search on a website, the server generally returns a page of links to site content. The industry does not consider the first page of this search result to be a valid page impression, however, it may be advantageous for sites to record the number of searches made. 

Stream

"An indicator of a media file stream sent to a valid User as a result of that User's request being received by the server."

Streamed media represent a significantly different medium than html content with a different potential for advertising and so are generally counted separately. Streamed media are not considered to be countable as a valid page impression.
A stream is recorded by either counting the download of the media itself or the opening of a media player used to receive the stream. For certain streamed media, it is possible to calculate many metrics such as stream duration, data rate, unique users,and even pauses and skipped content.

Ad impression

"A file or a combination of files sent to a valid User as an individual advertisement as a result of that user's request being
received by the server."

In a similar vein to the page impression metric, an ad impression is considered to be the result of a single request by a browser.  If a collection of images are returned from a single browser request, a single ad impression is counted.

Ad click

"The activation of a hypertext link on an Ad Impression by a valid Ad User."

Ad user

"The total number of unique combinations of a valid identifier contributing to the Ad
Impressions served. Sites may use (i) IP+UserAgent, (ii) Cookie, and/or
(iii) Registration ID."

A complete list of industry-agreed metrics for the UK and Ireland is available at the ABC Electronic website.

In undergoing a site audit to produce these measurements, there are a number of considerations.