mutation
A mutation is a permanent change in the sequence of letters that spells out the genetic code. Imagine a recipe book copied by hand over and over: now and then a word gets miscopied, a line gets skipped, or an extra paragraph slips in. Most such slips are harmless, a few ruin the dish, and once in a great while one makes the recipe better.
More precisely, a mutation is any alteration to the DNA sequence that is stable and can be passed on when the cell or organism reproduces. Changes range from a single swapped base to the gain, loss, or rearrangement of whole chromosomal regions. The change is heritable in the cellular sense: once it is fixed in the DNA, every descendant of that cell carries it.
Mutations are the ultimate source of all genetic variation, the raw material on which natural selection and genetic drift act. Whether a particular mutation is harmful, neutral, or beneficial depends entirely on its effect on the gene product and on the environment; a change that is bad in one setting can be advantageous in another. The word itself carries no value judgement.
In everyday speech “mutation” often sounds sinister, but most mutations are neutral and silent. The dramatic phenotypes from comic books are not how real mutations usually behave.