Mutation

A mutation is a sudden heritable change in the genetic material, i.e. a change in the nucleotide (base) sequence of DNA, such as substitution, insertion or deletion of bases.
A mutagen is the agent (e.g. UV rays, X-rays, chemicals) that causes mutation.
Recombination is the reshuffling of existing genes, and translation is protein synthesis.
So a change in the sequence of nucleotides in DNA is called mutation.

Most mutations disturb the normal working of genes, so they are harmful.
Most mutant alleles are also recessive: one normal allele in a heterozygote usually makes enough of the normal product to mask the mutant effect.
Beneficial mutations are rare.
So most mutations are harmful and recessive.