as a preposition (followed by a noun or number): There are piles of books under my desk. ♦ The total cost of the project is just under £2.2 million. ♦ The technology has been under development since the early 1980s.
as an adverb (without a following noun): Jump into the water and see how long you can stay under. ♦ Children aged five and under can get in for half-price.