Merriam-Webster Collegiate Dictionary - under

 I. adverb  Etymology: Middle English, adverb & preposition, from Old English; akin to Old High German untar ~, Latin inferus situated beneath, lower, infra below, Sanskrit adha  Date: before 12th century  1. in or into a position below or beneath something  2. below or short of some quantity, level, or limit $10 or ~ — often used in combination ~staffed  3. in or into a condition of subjection, subordination, or unconsciousness put the patient ~ for surgery  4. down to defeat, ruin, or death businesses going ~  5. so as to be covered buried ~ by the avalanche  II. preposition  Date: before 12th century  1. below or beneath so as to be overhung, surmounted, covered, protected, or concealed by ~ sunny skies a soft heart ~ a stern exterior ~ cover of darkness  2.  a. subject to the authority, control, guidance, or instruction of served ~ the general ~ the terms of the contract a program that runs ~ any operating system  b. receiving or ~going the action or effect of ~ pressure courage ~ fire ~ the influence of alcohol the image of a point ~ a mapping ~ oath  3.  a. within the group or designation of ~ this heading  b. having as name or title traveling ~ an alias  4. less or lower than (as in size, amount, or rank); especially falling short of a standard or required degree ~ the legal age ~ par  III. adjective  Date: 13th century  1.  a. lying or placed below, beneath, or on the ventral side — often used in combination ~lip  b. facing or protruding downward  2. lower in rank or authority ; subordinate  3. lower than usual, proper, or desired in amount, quality, or degree — often used in combination an ~-dose of medicine