That's the "~" character, usually far left next to ! and 1. Some folks call it "squiggle" but it isn't.
It's often shorthand for your home directory: cd ~/bin for example, but remember that's a function of your shell, not your OS.
It's used as the escape character in cu and ssh: ~. exits, ~! escapes to a shell and more: see the man page. Linux people will have no idea what cu is, of course. Nor will a lot of BSD folk.
Tildes appear in vi when there are "empty" lines on your screen. See Why was '~' chosen to represent the home directory? and Wikipedia's entry too.
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