Question

what do you mean by lightning and Thunder​

Answer 1

lighting is the light travelling in the atmosphere ( that we usually call 'bijli') but thunder is its sound

Answer 2

Lightning:- The occurrence of a natural electrical discharge of very short duration and high voltage between a cloud and the ground or within a cloud accompanied by a bright flash and typically also thunder.

Thunder :- A loud rumbling or crashing noise heard after a lighting flash due to the expansion of rapidly heated air.