short for Terabyte (2^40 bytes). as in, "my 50GB hard disk isn't big enough for all my mp3s; I need at least 2TB!"
Incidentally, not that anyone cares, but my family used to have a cute little black cocker spaniel, who we got from a dog breeder when he was retired. His first owners had named him "Tar Baby", and who were we to change it? (It certainly did fit him, in both appearance and personality.) Well, my mom, for some unknown reason, likes to call people by initials that aren't necessarily actually their initials; I am "BB", my sister is "EE", my dad is "IR", and Tar Baby became "TB". Interestingly, our cat, Fox (named after The X-Files character of the same name, after "Cerulean" was nixed) never did acquire initials... had my sister and I succeeded in getting him named "Cerulean Blue", he would have been "CB", I guess. Oh well...