Favored Class
Favored Class is a games mechanic native to Dungeons & Dragons 3rd edition, which has yet to resurface.
In a nutshell, Favored Class was developed from the principles of Racial Class Restrictions pioneered in Advanced Dungeons & Dragons: perhaps because of its origins, D&D has traditionally had an issue with pigeonholing races into classes based on what the designers thought were archetypical. For example, all gnomes will only use illusion magic if they study to become wizards, a dwarf would never use non-clerical magic, only a human has the purity of soul to be a paladin, and so forth.
Needless to say, this idea was hugely contentious amongst the fanbase.
In recognition of that, when Wizards of the Coast took over the D&D license and decided to produce a new edition of the game, they decided to junk that rubbish - at least, partially. Fearful of arousing too much grognard rage, they created the system of the Favored Class, which would encourage players to stay closer to these archetypical classes.
Working off of the new multiclassing rules, it basically functioned like this: apart from certain rare exceptions, all races had a favored class. If they had levels in this favored class, than either they needed to keep this class highest whilst multiclassing or suffer XP penalties, or levels in it simply did not count towards XP penalties, depending on whether you're talking 3.0 or 3.5.
The mechanic... wasn't exactly well-received. The whole idea of classes being favored bugged some fans, who complained that this didn't match their homebrew campaign's depiction of a given race, but most focused on the fact that it literally had no effect unless you were multiclassing.
So, the mechanic was cancelled with 4th edition's release, and doesn't look like returning any time soon.