#

Most commonly referred to as the pound symbol (also known as the "hash" symbol). Its proper name is the "octothorpe" or "octothorp": either spelling is acceptable. It is most commonly used to represent numbers, as in, "I'll answer #3."

In some versions of Unix and Linux, it is the system prompt. It has a variety of uses online, either as part of a cryptic password, or in creative renditions of ASCII art and smileys. 

The hashtag went mainstream when Twitter users began adding it to a word or phrase like #word when searching for tweets. 

It also means "net." Under the Linux operating system it is sometimes referred to as the "crunch" symbol as well, such as in the Linux distribution CrunchBang which gets its name from the first two characters of a bash, or alternate (ash, csh, etc.) shell script which, to be made interpreted by the interpreter, must begin with a line like the following: #!/path/to/script/interpreter For bash, this would be #!/bin/bash".