In other words, this is a relative or a friend or even a rival whom we really should have met, or at least heard about before, since the other characters would have us believe they have been crewing the same starship, working on the same project or sitting on the other side of the same classroom for years.
It comes about because the writers want to introduce a new character, but don't want to come up with a complex introduction where everyone meets them and learns what they're like- it's just plain easier (and lazier) to pretend that everyone automatically knows them, and the relationship is ready-made. " "Oh, this is my brother/sister whom I've never told you about," etc.
(In fact, the term "Retroactive Continuity" was originally coined to describe trope).
When he was 15, he was expelled from Stanbridge Earls School because of cutting class and entertaining a girl in his room. His first marriage to a pop icon ended in divorce after eight years of their togetherness.
Then for the second time he recently married an English model and the couple are living together.
Among the most clumsy versions is the "Long-Lost Daughter/Son." This trope can be especially disruptive if the new addition doesn't fit the tone of the series.
A subversion of this trope can come into play when a newly-introduced character actually there before, but the characters believe they were due to Fake Memories or a Cosmic Retcon.