Nick

Your IRC nick is the name others see you as when you speak in channels. For Example:

<TheRat> jesterina: How do I "link" pages?
<jerome> use the help guide newb
<jerome> :(
<TheRat> Blah
<mist|zZz> [[pagename]]
<mist|zZz> easiest way to learn how to do it, is to open an edit on an existing page, i found
<mist|zZz> oh, and to type categories right, that helps too

The nicks in the above conversations snippet are 'TheRat', 'jerome' and 'mist|zZz'.

Nicks are also used to perform irc commands, such as:

/kick #thechannel mist|zZz

which would kick mist|zZz from the channel.

Your nick is part of your hostmask.