WordReference Random House Learner's Dictionary of American English © 2015
one /wʌn/USA pronunciation adj. [before a noun] being or amounting to a single unit or individual or entire thing;
equal to the number 1:one child; only one piece of cake left.
being an individual instance, example, or member of a number, kind, or group indicated:one member of the team. of the same or having a single kind, nature, or condition:We are of one mind. (used to refer to an unspecified or imprecise day or time):one evening last week. [before a proper noun or name] (used to name a person otherwise unknown or not yet described):One John Smith was chosen. [before a singular countable noun] being a particular or only individual, item, or unit:She's the one person I can trust. (used to show strong feeling about the noun or adjective that follows) a or an:That is one smart dog.
n. [countable] the first and lowest whole number, being a cardinal number; a unit:Ten minus nine leaves one. a symbol of this number, as 1 or I. a single person or thing:Let's do one at a time. a one-dollar bill.
pron.  (used to stand for a person or thing of a number or kind that is about to be indicated):He is one of the Elizabethan poets. (used to stand for a person or thing that has just been mentioned or indicated, or is already understood from the context):The portraits are good ones. a person, or a personified being:Satan, the evil one. any person or thing; people in general:One shouldn't cry over spilled milk (= People in general, including the speaker, shouldn't get upset about things that can't be fixed.) idiom
    all one, the same:You can stay or go; it's all one to me. (= It doesn't matter to me which action you take.) as one: 
    • with complete agreement;
      unanimously:They voted as one.
    • all at the same time; in unison:We rose to our feet as one.
    at one: 
    • united in thought or feeling;
      attuned:to feel at one with the world.
    Idiomsone and all, everyone. Idiomsone by one, singly and following after another.

Collins Concise English Dictionary © HarperCollins Publishers::

one /wʌn/ determiner
  1. single; lone; not two or more
  2. (as pronoun): one is enough for now, one at a time
  3. (in combination): one-eyed, one-legged
  4. distinct from all others; only; unique: one girl in a million
  5. (as pronoun): one of a kind
  6. a specified (person, item, etc) as distinct from another or others of its kind: raise one hand and then the other
  7. (as pronoun): which one is correct?
  8. a certain, indefinite, or unspecified (time); some: one day you'll be sorry
  9. informal
    an emphatic word for a, an1: it was one hell of a fight
  10. a certain (person): one Miss Jones was named
  11. in one, all in onecombined; united
  12. all oneall the same
  13. of no consequence: it's all one to me
  14. at one ⇒ (often followed by with) in a state of agreement or harmony
  15. be made one(of a man and a woman) to become married
  16. many a onemany people
  17. neither one thing nor the otherindefinite, undecided, or mixed
  18. never a onenone
  19. one and alleveryone, without exception
  20. one by oneone at a time; individually
  21. one or twoa few
  22. one way and anotheron balance
  23. one with anotheron average
  1. an indefinite person regarded as typical of every person: one can't say any more than that
  2. any indefinite person: used as the subject of a sentence to form an alternative grammatical construction to that of the passive voice: one can catch fine trout in this stream
  3. archaic an unspecified person: one came to him
  1. the smallest whole number and the first cardinal number; unity
  2. a numeral (1, I, i, etc) representing this number
  3. informal a joke or story (esp in the one about)
  4. something representing, represented by, or consisting of one unit
  5. Also called: one o'clock one hour after noon or midnight
  6. a blow or setback (esp in the phrase one in the eye for)
  7. the Holy One, the One aboveGod
  8. the Evil OneSatan; the devil
Related prefixes: mono-, uni-
Etymology: Old English ān, related to Old French ān, ēn, Old High German ein, Old Norse einn, Latin unus, Greek oinē ace

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