Interbank Rate

See: Interbank.

It's the rate at which one bank lends to another.

Remember that the Fed lends money to banks initially. But they don't do it in one steady flow; in fact, Fed money comes out in waves, like the Sunday morning after New Years when you've had too much to drink.

So banks, finding themselves with more cash than they can loan out to would-be borrowers, ask to offload some of that dough to other banks who might be in more direct need of the dough.