C

Click Through Rate
The number of clicks on a link, as a percentage of the number of views of the link.

Cloaking
Cloaking is a method by which specific content is served up to the search engine spider that is different then what the normal surfer sees.

Clustering
In search engine search results pages, clustering is limiting each represented website to one or two listings.

Content-rich Doorway
A doorway page dressed up with graphics, navigation, and linked to from a site map so that it looks like a normal part of a Web site. The copy is written to rank for a single keyword expression.

Contextual Link Inventory
Contextual Link Inventory or CLI is text-link advertising that is displayed based on the relevance of surrounding content.

Contextual Link Inventory is mostly done on websites (via networks like Google Adsense) and via search engine results pages.

The ad server checks the content of pages or search queries (on SERPS) to find matching ads. The success of the Contextual Link Inventory depends on the quality of the content and the relevance of the content to the niche/theme or the core objectives of the web site. Ads that connect better to the pages content have higher click-trough rates then unrelated ads.

Conversion Rate
The key metric to evaluate the effectiveness of a conversion effort (accepting a free gift, setting an appointment), reflecting the percentage of people converted into buyers (or subscribers, or whatever action is desired) out of the total population exposed to the conversion effort. For Web sites, the conversion rate is the number of visitors who took the desired action divided by the total number of visitors in a given time period (typically, per month).

For example, the conversion rate of visitors that subscribe to a newsletter = number of visitors divided by number of subscribers. If a website has 10,000 visitors and 500 subscribe, the conversion rate equals 1 in 20.

Cost Per Click
The amount an advertiser pays an ad host (such as a site owner, search engine or ad network) each time a visitor clicks on the advertiser's link.

Cost Per Thousand
A common term from traditional media advertising; online it refers to the cost per thousand people viewing an ad or listing (regardless of whether they respond to it).

Crawl Page
A document consisting of links to other pages, provided for the sole purpose of giving crawlers (robots) links to follow. Spammers used to submit these puppies to the search engines en masse.