There's not much difference between the definition of a seat and a bench.
A seat is a place to sit and a bench is a long seat.
My idea of a bench was probably embedded in my memory after a P.E. Lesson, when I
was a youngster. Working in pairs, as instructed, we approached a high stack of what
seemed to be these long pieces of two-tone wood.
Taking hold of each end (and you only do this the once), concealed folding legs dropped
down against your shins. A Gym bench is a nasty piece of equipment, unless you can
see the legs. Handling these benches, which turned up in the school dining room,
needed care. If you position your hands near the ends, with your thumbs on the top
and your fingers down along the longer edge, you grab the folding legs before they
have the chance to attack the thin skin on your shins. Use a really firm grip or
a slight movement of the flapping legs nips your fingers.
Once you and your pals knew the secret, you didn't tell others. On one occasion,
these two inexperienced lads needed a bench for their dining table. They yelled when
the folding legs banged against their legs, but for whatever reason, they let go.
They did this “standing in one place jig” as the bench bounced from their plimsoled
feet. The noise disturbed the silence (and once upon a time, school dining rooms
were places to eat not talk - “Open your mouth only to put food in, boy!”)
The lads were put in detention, “For prancing about, showing off, trying to damage
school equipment, scratching the floor, disturbing the dining room occupants and
shouting out.” That's when we developed the silent laughter routine.
So what is a bench?
My definition is a bench is a long seat fit for several people, without a back and
without arm rests. Other definitions include a back and/or arm rests.