1) An electronic circuit that reverses ("flips") its state when a pulse is administered to it.
2) A behaviour similar to the aforementioned circuit's. Reversing your stance, depending on input.
3) A kind of open shoe.
1) A register has to store and erase values as needed, so you must use flip-flops to implement one.
2) Politicians are extremely prone to flip-flopping, especially when votes are at stake.
3) I will buy some matching red flip flops to go with my red T-shirt.