Why "Bistable" means the circuit has two stable output states and will stay in either one until an input tells it to change. A flip-flop does exactly that: it latches a 0 or a 1 and holds it, which is why flip-flops are the basic one-bit memory element and the building block of counters and frequency dividers. Gates, by contrast, have no memory; their output is determined moment by moment by the present inputs.
Watch out AND and OR gates are combinational logic, so their outputs follow the inputs instantly with nothing stored, and a bipolar amplifier is an analog device whose output varies continuously rather than resting in two discrete states.
Bi-stable = two stable states = flip and flop. Flip-flops remember, gates just react.