something used instead of a person's realname that either makes it shorter, is a joke, or is a word that describes their personality

A nickname is a term of familiarity between a person or a thing/another person.

A name given to a person that is typically a shorter version of their existing full name. Nicknames can also be given to a person based on personality traits or other defining attributes of that person. The "nickname" usually derives from a friend, family member, or sometimes enemy calling a specific person a name not their own. The "nickname," if catchy enough, will stick and that person will begin being referred to by their nickname moreso than their actual full name.

The term, "nickname" originated from the the time of St. Nicholas when people had a particular difficulty with phonetics. The commoners had difficulty saying Nicholas, so they began calling him: "St. Nick", The term, "nick" also referrs to something being chipped off, such as: "Thou nicked off a piece of thine rock to make it fit in thy dwelling." Over the centuries, nickname became the term for a shortening of someone's full name, like a part of it being nicked off.

A name that someone gives you because they don't like your real name

The self you wish you could be based on what a name sounds like or means.