"Fam" was a term that the Thirteenth Doctor had taken to calling her companions.
She initially considered the name along with "troops", "team", and "gang". (TV: The Woman Who Fell to Earth, Arachnids in the UK) Despite later settling with the term team TARDIS, (TV: Arachnids in the UK) she brought up the possibility again, which was rejected by her companions. (TV: The Witchfinders) The next time she brought it up, Yasmin Khan expressed some liking to it, but Ryan Sinclair and Graham O'Brien were still not convinced. (TV: The Battle of Ranksoor Av Kolos)