least

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Lowest in importance or rank.
• adjective Smallest in magnitude or degree.
• adjective Slightest or tiniest.
• adverb To or in the lowest or smallest degree.
• noun One that is the lowest or smallest in importance, rank, magnitude, or degree.
• idiom (at least) According to the lowest possible assessment; not less than.
• idiom (at least) Regardless of what has occurred or will occur; in any case.
• idiom (at least) Used to emphasize a somewhat positive aspect following a negative one.
• idiom (in the least) At all.
• idiom (least of all) Particularly not.

from The Century Dictionary.

• In the smallest or lowest degree; in a degree below all others: as, to reward those who least deserve it.
• Smallest; little in size or degree, etc., beyond all others: answering as superlative to little.
• An obsolete spelling of lest.

from the GNU version of the Collaborative International Dictionary of English.

• adverb In the smallest or lowest degree; in a degree below all others.
• conjunction obsolete See lest, conj.
• adjective Smallest, either in size or degree; shortest; lowest; most unimportant
• adjective at the least estimate, consideration, chance, etc.; being no less than; hence, at any rate; at all events; even. See However.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adverb Used for forming superlatives of adjectives, especially those that do not form the superlative by adding -est.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun something that is of no importance
• adverb used to form the superlative
• adjective the superlative of `little' that can be used with mass nouns and is usually preceded by `the'; a quantifier meaning smallest in amount or extent or degree