Wikipedia:

On Unix-like operating systems (including BSD, GNU/Linux and Mac OS X), tilde often indicates the current user's home directory: for example, if the current user's home directory is /home/bloggsj, then cd, cd ~, cd /home/bloggsj or cd $HOME are equivalent. This practice derives from the Lear-Siegler ADM-3A terminal in common use during the 1970s, which happened to have the tilde symbol and the word "Home" (for moving the cursor to the upper left) on the same key.

http://rollmops.wordpress.com/2006/05/01/vintage-computer/

http://geekblog.oneandoneis2.org/index.php/2012/09/30/to-understand-the-command-line