What is the unit "Th" as in Th/s? I keep seeing this everywhere, but it seems impossible to search. What does Th stand for? What does it mean? How do you calculate the Th?

For example, "The 13.5 TH/s is $2773.28 US". What does that even mean?
oops, I should make it an answer, not a comment:

Tera Hashes - a kind of a speed indicator, how fast the mining machine can calculate. "The 13.5 TH/s is $2773.28 US" would mean, a machine with such a power can generate this amount of Dollars...