Used in an ironical sense to mean legitimate, and therefore, in reality, spurious and not at all legitimate. Assumes common knowledge of the inherent Simpsons reference.

I believe this term originated in the Animated Television Series "The Simpsons"

Expression used to legitimize another expression of dubious existence. Used in the ironic sense in that it defends the questionable use of an illigitimate or slang word with another made-up expression. The intent of the phrase is generally used in conjunction with insults to high talkers, english majors and otherwise insufferable people who feel the need to correct others. Often accompanied by a punch to the stomach.

Cromulence is when a word/situation/subject seems realistic and plausible, but is actually entirely made up.

(Adj.) Used to describe a dubious or made up word, term, or phrase that is entirely plausible because it makes logical sense within existing language conventions.

Ironically, the word comulent itself is not cromulent. (See incromulent)

Origin: Television writer David X. Cohen for The Simpsons episode 3F13, "Lisa the Iconoclast".

Not simply used to refer to a frequently used word as per the use in "a perfectly cromulent word" but may be better defined as germane, or apt, synonomous with appropriate, or fitting.
This defintion suits both uses in The Simpsons' famous episode.

1. To be hanging out with someone but not officially going out with them.

2. An in between not going out with someone and going out with them.