When someone calls someone else up and is like, "Hey, let's have sex." Usually it's not said that upfrontly, though. Usually it's done over text, initiated with some flirting and a lot of winky faces.
However, according to the region, booty call can mean something quite different. In certain towns such as Provo and Rexburg, where sex before marriage is highly discouraged, some of the residents loosely use the term in place of "NCMO".
I'm feeling a bit lonely tonight. Time to make a booty call.
Provo/Rexburg/other conservative towns use:
Guy 1: So my ex started texting me again the other night.
Guy 2: Seriously? What did she want?
Guy 1: She said she missed me and that she wanted to hang out.
Guy 2: Dude, total booty call.
Guy 1: Yeah, we ended up making out for like 15 whole minutes.
Guy 2: Get itt.