Term used by amateur flight "simmers" who typically watch certain disgraceful Youtubers. It is used to describe unrealistically smooth aircraft landings in a flight simulator or even real life, typically under a descent rate of 75 feet per minute. People who use this term are often frowned upon due to the fact that "butter" is usually used as a way to flex or appear as a tryhard.
Look at that butter! The aircraft landed so smoothly!
Did you see that Airbus A330 butter? I bet the passengers never knew when they actually landed.
OMG that was such a smooth butter!
noun
1. a point or level beyond which someone does not have the intestinal fortitude to digest butter.
2. a restriction on the amount of butter permissible or possible to safely eat.
Eric: Hey, should we do crab legs for lunch, or will that give you bubble guts?
Blair: That would definitely put me over my butter limit; and I don't want to be on the toilet all night.