Liminal \LIM-uh-nl\ , adjective;
1. Relating to the point beyond which a sensation becomes too faint to be experienced
According to Etymonline.com:
Dating to 1884, this rare word comes from Latin limen, which means "threshold".
It's always interesting to find out about a word like this. I bet that most of us have at least heard of the word subliminal, and any native speaker knows that sub- is a prefix, so it makes perfect sense that liminal is a word. However, that line of thinking doesn't always apply (oh, the complexity of language) so you don't necessarily think about the viability the root word. Except, of course, in the epic 90's classic** "Clueless" where it was asked, "You can be overwhelmed and you can be underwhelmed, but can you just be whelmed?"
*Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Thursday, December 16