an alternative name taken by or given to a person, thing, or place, usually descriptive in nature and humorous or familiar in spirit, which may be used in addition to or in the place of the original name;
e.g. You might address someone by a nickname when you become close friends with them.
(verb)
to assign an alternative name to a person, place, or thing, or to call them by such an alternative name;
e.g. They thought about what they would nickname their new pet.