# What is the name of this “set similarity” function?

What is the name of this function, expressing the "similarity" of two sets $S_1$ and $S_2$?

$$f(S_1,S_2) = {|S_1 \cap S_2| \over |S_1 \cup S_2|}$$

It's such a basic function that it surely have a name, but I can't find it.