A mutation is a genetic change that causes new and different characteristics, like the mutation on the dog's DNA that makes its tail shorter than its ancestors' tails.
Mutation comes from the Latin word mutationem meaning "a changing." You might recognize this root in related words like mutate, mutable, and mutant. We often think of mutations as changes in an animal's genetic structure, but there are other kinds of mutations. For example, if an architect designs an award-winning building, her later designs that are similar might be called mutations of that one.