Call To Action
A call to action is copy used in advertising to encourage a person to complete an action as defined by the advertiser. Example: "Click here", "Buy Now", "Enter Now" or "Click to download".  

CGI (Cgi-bin)
A "virtual" directory contained in URLs indicates a CGI (Common Gateway Interface) script is in use. It alerts the spider that the page is dynamic.

Cloaking
Showing content to search engines and displaying a different page to your site visitors. It’s generally considered to be search engine spamming.

Clustering
In search results the listings from any individual site are typically limited to a certain number and grouped together to make the search results appear neat and organized and to ensure diversity amongst the top ranked results. Clustering can also refer to a technique which allows search engines to group hubs and authorities on a specific topic together to further enhance their value by showing their relationships.

CMS (Content Management System)
A tool sold to clients to allow them to update and add information to their website.
No coding skills are required; it’s as simple as using MSWord.

Content Integration
Advertising woven into editorial content or placed in a special context on the page. Also known as sponsored content.

Cookie (HTTP cookies)
Cookies are small pieces of (tracking) text, stored by a user's web browser, that contain the user's settings, shopping cart contents, or other data used by websites.
While the web site is being accessed, data in the visitor's cookie file can be stored or retrieved. Mostly cookies are used as unique identifiers (i.e. user IDs or session IDs) to isolate a visitor's movements from others' during that visit and subsequent visits. Other data that may get stored in a cookie include an order number, email address, referring advertiser, etc

Content (copy, text)
The main body of text a person can read on a web page via a web browser.
Advertising, navigation, branding, images, are not considered to be content.

CPA
Cost Per Action or Cost Per Acquisition

CPC
Cost Per Click

CPM
Cost Per Thousand

CSS
Cascading Style Sheet

Clickthrough 
The action of clicking an ad element and causing a redirect to another web page.

Clickthrough rate 
the rate at which people click on a link such as a search engine listing or a banner ad. Studies show that clickthrough rates are six times higher for search engine listings than banner ads

Conversion 
the act of converting a web site visitor into a customer or at least taking that visitor a step closer to customer acquisition (such as convincing them to sign up for your e-mail newsletter

Conversion rate 
the rate at which visitors get converted to customers or are moved a step closer to customer acquisition

Cost Per Action (CPA) 
the cost incurred or price paid for a specific action, such as signing up for an email newsletter, entering a contest, registering on the site, completing a survey, downloading trial software, printing a coupon, etc

Cost Per Click (CPC) 
the cost incurred or price paid for a clickthrough to your landing page

Cost Per Lead (CPL) 
Pricing based on the number of new leads generated

Cost Per Order (CPO) 
Pricing based on the number of orders received as a result of your ad placement. Also known as cost-per-transaction

Cost Per Sale (CPS) 
Pricing based on the number of sales transactions your ad generates. Since users may visit your site several times before making a purchase, you can use cookies to track their visits from your landing page to the actual online sale. Also known as cost-per-acquisition or pay-per-sale. 

Cost Per Thousand (CPM) 
The cost incurred or price paid for a thousand impressions

Counter 
a simple program which tracks the total number of webpage impressions

Crawl Frequency
How frequently a website is crawled. Sites which are well trusted or frequently updated may be crawled more frequently than sites with low trust scores and limited link authority. Sites with highly artificial link authority scores (ie: mostly low quality spammy links) or sites which are heavy in duplicate content or near duplicate content (such as affiliate feed sites) may be crawled less frequently than sites with unique content which are well integrated into the web.

Crawler 
see "spider"

CSS 
Cascading Style Sheet - used to control the design of website

CSS
Cascading Style Sheets is a method for adding styles to web documents.
Note: Using external CSS files makes it easy to change the design of many pages by editing a single file. You can link to an external CSS file using code similar to the following in the head of your HTML documents

<link rel="stylesheet" href="http://www.seobook.com/style.css" type="text/css" />

CTR - Click Through Rate 
Click Through Rate is a measure of the number of clicks received from the number of ad impressions delivered.
The formula to calculate CTR is:
“number of clicks” / “number of ad impressions” x 100

Custom error page 
You can customize the content and the look-and-feel of the default page that is displayed on your web server when a 404 File Not Found error occurs. A good 404 error page has a friendly message explaining that the page they requested doesn't exist at the location, a site map to encourage the user to continue exploring the site, a search box so the user can conduct a search, and a look-and-feel that matches the rest of the site, including navigation of course. Creating a custom 404 error page not only helps keep visitors in your site, it is also an important part of the search engine optimization process. Inevitably pages on your site will get moved and removed over time. When a search engine spider returns to your site to reindex those now non-existent pages, they will have a set of links to explore in the form of the site map on the custom 404 page. You can test for whether a site has a custom 404 error page by trying to access a web page with a nonsense filename after the domain name in the web site address. For example: www.yourcompany.com/blah contextual advertisement Advertising which is related to the content.

Conversion (goal)
Achievement of a quantifiable goal on a website. Add clicks, sign ups, and sales are examples of conversions.

Conversion rate
Percentage of users who convert - see conversion.

CPC Cost Per Click
The rate that is paid per click for a Pay Per Click Advertiser

CPM (Cost Per Thousand impressions) A statistical metric used to quantify the average value / cost of Pay Per Click advertisements. M - from the Roman numeral for one thousand.

Crawler (bot, spider)
A program which moves through the worldwide web or a website by way of the link structure to gather data.