The 301 status code means that the requested URL has Moved Permanently and has been assigned a new URL. It is best to use 301 Redirects when multiple copies of the same document are on different URLs. This will ensure that duplicate content is removed from the site and that each and every unique page will only have one URL.

The 302 status code means that the requested document has been found however it resides temporarily under a different URL. Since a permanent redirect has not been used, the client should continue using the URL for future requests.

The 400 status code means a Bad Request which means that the server is not able to understand the document requested due to an incorrect syntax. The user is required to modify the request.

The 401 status code means Unauthorized. This server requests user authentication before fulfilling the document request.

The 403 status code means Forbidden. The server understood the request however, it refuses to fulfill it. The webmaster may alert the user as to why the request has been denied. If the organization does not wish to provide this reason then a 404 (Not Found) status code can be displayed instead.

The 404 status code means that the document is Not Found. This means that the client was able to communicate with the server however, the requested document could not be found.

The 410 status code means that the requested document is intentionally gone, is no longer available, and there is no forwarding address. The 410 status code is usually used for limited display documents such as documents or pages for promotion.

The 500 status code error message states that there was an internal server error which has prevented the document from being fulfilled.

The 501 status code message is displayed when the server does not recognize the document request method. The server is incapable of fulfilling the request and states that the request was not implemented.

Above the fold is the area in an email or web page where the content can be viewed before scrolling down. Some define it as the location of an ad at the top of a screen. Due to banner blindness, the usual location of ads does not perform as well as ads that are integrated in the content of a site.

Absolute link is a link that shows the full URL of the page being linked to. Absolute links are preferred over relative link paths due to canonicalization and other hijacking issues.

Accessibility is the practice of creating sites that are more accessible or usable for disabled persons particularly the blind. Accessible sites tend to have higher rankings in search results than inaccessible sites primarily because search engines are essentially blind and heavily rely on texts.

An acquisition is a person who not only browses a site but also subscribes to the site, purchases a product from the site, or leaves his or her details on the site - depending on the goals of the website.

The acquisition cost is the total cost of advertising divided by the number of the visitors of the site or the number of customer the site was able to acquire.

An ad broker is a person who sells advertising space on a website.

Ad Centre is a cost per click ad network that is owned by Microsoft.

The ad inventory is the total number of potential views that a page has. This is used for advertising purposes.

Adjacency is the relationship between words in a search engine query. Search engines give sites a higher rank when the words that are searched for appear side by side in the body of text on a website compared to words that are separated by other words that are not included in the search query.

Adjacent searching is also known as proximity search. This is a technique to check the position of terms and match terms that are within the specified distance between keywords. This is often used to get better results in a search query by removing irrelevant matches.

AdSense is a contextual ad based program by Google. It is a way of earning money by displaying Google ads in the pages of your site.