CIA (Complex Interface Adapter, though I suspect this acronym was invented retroactively), Mostech 6526, is a chip in Commodore 64 that handles I/O.
Actually, there's two of these chips on C64 motherboard. CIA 1 ($DC00 and onward) handles keyboard and joysticks, and CIA 2 ($DD00 and onward) handles serial bus and user port.
CIA 1 also controls the timer interrupt (IRQ interrupt), which also makes it a pretty interesting chip from programmer's point of view...