| Word | Description |
| Cable modem | A modem that transmits data over cable TV networks. |
| Cache | A memory storage area for regularly accessed or recently used data. |
| Certification Authority | A trusted third party that issues digital signatures. For example VeriSign and Thawte. |
| CGI (common gateway interface) | A bi-directional protocol that allows the transmission of data within an active or dynamic environment (that is, between a server and a program designed to accept and return data of this type). |
| CGI scripting language | A language that is used to create CGI programs or scripts, e.g. C, Visual Basic, Java. |
| Clicks & mortar | A business model where an organisation exists both physically and as an online presence. |
| Client/server | A distributed system architecture where client systems are connected to server systems. |
| Communication software | A software program that runs on personal computers, and allows the computer to communicate with other computers via a modem and phone lines. |
| Cookie | Information that is sent from a web site to a computer's hard disk in the form of a text file. Cookies contain all types of information, such as registration information and user preferences, which is used by the web site that sends the cookie. |
| Cryptography | An encryption process that encodes and decodes data or information to protect it from unauthorised or unintended use. |