1. A storage area that keeps frequently accessed data or program instructions readily available so that you don't have to retrieve them repeatedly from slow storage. Caches improve storage by storing data or instructions in faster sections of memory and by using efficient design to increase the likelihood that the data needed next is in the cache. From QUECID

2. A small fast memory holding recently accessed data, designed to speed up subsequent access to the same data. Most often applied to processor-memory access but also used for a local copy of data accessible over a network etc. From Linux Guide @FirstLinux

3. In general computer science, the word cache means simply to keep things around in case they are used again. For example, when you log onto your system, your username and password are stored in a cache in memory, because they are repeatedly used by the system every time you access a resource. Key point: Sometimes systems can be exploited through the cache. Examples are: HTTP proxy servers Companies use these so that thousands of users can share a single Internet connection. They store recently used webpages so that when multiple users access the same web-site, the proxy server only has to go across the link once in order to fetch the page for all the users. A never ending series of bugs leads to conditions whereby when one user logs into a website, other users can see that first user's data. Web-browser history/file cache Once a hacker breaks into a machine, he/she can view the history cache (list of URLs) or file cache (the actual contents of the web-sites) in order to spy on where the user has been. Embarrassing, inadvertent disclosure of this information by users with certain surfing habits is common. Web-browser cookie cache Lots of web-sites store passwords within cookies, so that stealing somebody's cookie information will allow a hacker to log in as that user. From Hacking-Lexicon