What is mutation ?

Dear student

The meaning of mutation is alteration or change.
I suppose you are asking for gene mutation.
It means the change in nucleotide sequence of a gene which may or may not result in any observable phenotypical change.
like due to some agent, in a gene, nucleotide A is mutated by T, this may result in formation of a wrong protein or no protein at all, it happens if the  gene is coding.
The agents which can cause mutation to occur can be chemical compounds or radiations.
These are broadly classified as base, insertion , deletion mutation.
 
regards