under

American Heritage® Dictionary of the English Language, Fourth Edition

1. prep. In a lower position or place than: a rug under a chair.
2. prep. To or into a lower position or place than: rolled the ball under the couch.
3. prep. Beneath the surface of: under the ground; swam under water.
4. prep. Beneath the assumed surface or guise of: traveled under a false name.
5. prep. Less than; smaller than: The jar's capacity is under three quarts.
6. prep. Less than the required amount or degree of: under voting age.
7. prep. Inferior to in status or rank: nine officers under me at headquarters.
8. prep. Subject to the authority, rule, or control of: under a dictatorship.
9. prep. Subject to the supervision, instruction, or influence of: under parental guidance.
10. prep. Undergoing or receiving the effects of: under constant care.
11. prep. Subject to the restraint or obligation of: under contract.
12. prep. Within the group or classification of: listed under biology.
13. prep. In the process of: under discussion.
14. prep. In view of; because of: under these conditions.
15. prep. With the authorization of: under the monarch's seal.
16. prep. Sowed or planted with: an acre under oats.
17. prep. Nautical Powered or propelled by: under sail; under steam.
18. prep. During the time conventionally assigned to (a sign of the zodiac): born under Aries.
19. adv. In or into a place below or beneath: struggled in the water but then slipped under.
20. adv. In or into a subordinate or inferior condition or position.
21. adv. So as to be covered or enveloped.
22. adv. So as to be less than the required amount or degree.
23. adj. Located or situated on a lower level or beneath something else: the under parts of a machine.
24. adj. Lower in rank, power, or authority; subordinate.
25. adj. Less than is required or customary: an under dose of medication.
26. idiom. out from under Informal Having gotten free of worries or difficulties: Credit counseling helped us get out from under.

Century Dictionary and Cyclopedia

1. Below; beneath: expressing position with reference to that which is above, whether in immediate contact or not, or which towers aloft, surmounts, covers, or overtops: as, all under heaven; under the earth or the sea; under the surface; under the table; to take shelter under a tree; to live under the same roof; to hide a thing under a heap of straw; to hide one's light under a bushel; to overhear a conversation under one's windows.
2. In or at a place, point, or position that is lower than; further down than; immediately below: as, to hit a man under the belt; to have pains under the arms.
3. In the position or state of, or while bearing, supporting, sustaining, receiving, suffering, undergoing, or the like: as, to sink under a load; to act under great excitement.
4. Inferior to in point of rank, dignity, social position, or the like.
5. Inferior to or less than, with respect to number, amount, quantity, value, age, etc.; falling short of; in or to a less degree than; hence, at, for, or with less than: as, it cannot be bought under $20.
6. Of sounds, inferior to, in pitch.
7. Subject to. In a position of submission or subordination to.
8. Liable or exposed to: as, under fire; under the penalty of fine or imprisonment.
9. Subject to the government, rule, command, direction, orders, guidance, or instruction of: as, to serve under Wellington; I studied under him; to sit under a favorite preacher.
10. Subject to the influence or operation of; actuated by.
11. In accordance with; in conformity with: as, to sell out under the rule.
12. Bound by: as, to be under bonds, or a vow.
13. In: with reference to circumstances.
14. In: with reference to category, division, section, class, etc.: as, to treat several topics under one head.
15. In course of: as, to be under treatment, or under discussion.
16. In the form or style of; by the appearance or show of; with the character, designation, pretense, pretext, or cover of.
17. During the time or existence of: said especially of rulers and their period of rule: as, Christ suffered under Pontius Pilate; the Armada was destroyed under the reign of Elizabeth; the American revolution broke out under the administration of Lord North.
18. With the sanction, authorization, permission, or protection of: as, under favor; under leave; under protection, etc.
19. [The preposition under in adverbial phrases often coalesces with its noun to form an adverb, from which the adjective or noun may be derived: as, under ground, ⟩ underground, adverb, ⟩ underground, a.; under hand, ⟩ underhand, adverb, ⟩ underhand, adjective; so underboard, underearth, underfoot, etc. Such forms are not true compounds, but are coalesced phrases, like aground, aboard, afoot, etc.]
20. In a state of subjection.
21. Nautical, directly under the bow: said of an anchor when the chain is up and down.
22. In a lower place; in a lower, subject, or subordinate condition or degree. The adverb under is much used in composition— With verbs and participles, and some nouns, indicating inferiority of place, ‘below, from below, on the lower part or surface,’ as in underbrace, underlay, undermine, underpin, underprop, etc.; indicating insufficiency, ‘insufficiently, imperfectly, below the required standard,’ as in underbred, underdone, underpaid, underrate, understate, etc.
23. Lower in position; situated beneath: opposed to upper: as, the under side; the under mandible.
24. Lower in rank or degree. See under, adverb, note .
25. Of sounds, lower in pitch.

Wiktionary

1. prep. In a lower level than.
2. prep. As a subject of; subordinate to
3. prep. Less than
4. prep. Below the surface of
5. prep. figuratively in the face of; in response to (some attacking force)
6. adv. In a way lower or less than
7. adv. In a way inferior to
8. adj. Being lower; being beneath something.

GNU Webster's 1913

1. prep. Below or lower, in place or position, with the idea of being covered; lower than; beneath; -- opposed to over
2. prep. Denoting relation to some thing or person that is superior, weighs upon, oppresses, bows down, governs, directs, influences powerfully, or the like, in a relation of subjection, subordination, obligation, liability, or the like; as, to travel under a heavy load; to live under extreme oppression; to have fortitude under the evils of life; to have patience under pain, or under misfortunes; to behave like a Christian under reproaches and injuries; under the pains and penalties of the law; the condition under which one enters upon an office; under the necessity of obeying the laws; under vows of chastity.
3. prep. Denoting relation to something that exceeds in rank or degree, in number, size, weight, age, or the like; in a relation of the less to the greater, of inferiority, or of falling short.
4. prep. Denoting relation to something that comprehends or includes, that represents or designates, that furnishes a cover, pretext, pretense, or the like.
5. prep. Less specifically, denoting the relation of being subject, of undergoing regard, treatment, or the like.
6. adv. In a lower, subject, or subordinate condition; in subjection; -- used chiefly in a few idiomatic phrases.
7. adj. Lower in position, intensity, rank, or degree; subject; subordinate; -- generally in composition with a noun, and written with or without the hyphen.

WordNet 3.0

1. adv. below some quantity or limit
2. adj. lower in rank, power, or authority
3. adv. down below
4. adv. in or into a state of subordination or subjugation
5. adj. located below or beneath something else
6. adv. into unconsciousness
7. adv. below the horizon
8. adv. further down
9. adv. down to defeat, death, or ruin
10. adv. through a range downward