Wikipedia quotes Marcus Hammarberg:

> The basic concept of mob programming is simple: the entire team works as a team together on one task at the time. That is: one team – one (active) keyboard – one screen (projector of course).

https://en.m.wikipedia.org/wiki/Team_programming#Mob_program...

The website from this hn submission seems last updated 7 years ago, perhaps the term was more popular back then.