Real Dictionary

under
If a person or thing is under something, they are at a lower level than that thing, and may be covered or hidden by it.
In a place such as a sea, river, or swimming pool, if someone or something is under the water, they are fully in the water and covered by it.

under in American English
in, at, or to a position down from; lower than; below
preposition: beneath and covered by

under in British English
adverb: below; to a position underneath something
preposition: directly below; on, to, or beneath the underside or base of

under-
prefix: Under- is used to form words that express the idea that there is not enough of something. For example if people are underfed, they are not getting enough food.

under- in American English
in, on, to, or from a lower place or side; beneath or below
prefix: a prefixal use of under, as to indicate place or situation below or beneath (underbrush; undertow); lower in grade or dignity (undersheriff; understudy); of lesser degree, extent, or amount (undersized); or insufficiency (underfeed)

under- in British English
prefix: below or beneath