a.k.a. pound sign, hash key, hashtag, number sign, square key, gate, hex sign, net
The pound symbol (also known as the "hash" symbol) is most commonly used to represent numbers, as in, "I'll answer #3." In some versions of Unix, it is the system prompt. It has a variety of uses online, either as part of a crypticpassword, or in creative renditions of ASCII art, or smileys.