Question

what is wave? which wave cannot travel in.which medium.

Answer 1

In physics, a wave is a disturbance that transfers energy through matter or space, with little or no associated mass transport. Waves consist, instead, of oscillations or vibrations of a physical medium or a field, around relatively fixed locations.
longitudinal wave cannot travel in vacuum
and transverse wave does not need a medium to travel

Answer 2

A wave is a disturbance that transfers energy through matter or space with little or no associated mass transport.