one

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

• adjective Being a single entity, unit, object, or living being.
• adjective Characterized by unity; undivided.
• adjective Of the same kind or quality.
• adjective Forming a single entity of two or more components.
• adjective Being a single member or element of a group, category, or kind.
• adjective Being a single thing in contrast with or relation to another or others of its kind.
• adjective Occurring or existing as something indefinite, as in time or position.
• adjective Occurring or existing as something particular but unspecified, as in time past.
• adjective Informal Used as an intensive.
• adjective Being the only individual of a specified or implied kind.
• noun The cardinal number, represented by the symbol 1, designating the first unit in a series.
• noun A single person or thing; a unit.
• noun A one-dollar bill.
• pronoun An indefinitely specified individual.
• pronoun An unspecified individual; anyone.
• idiom (at one) In accord or unity.
• idiom (one and all) Everyone.
• idiom (one by one) Individually in succession.

from The Century Dictionary.

• Alone; only.
• Being but a single unit or individual; being a single person, thing, etc., of the class mentioned; noting unity: the first or lowest of the cardinal numerals.
• Being a single (person or thing considered apart from, singled out from, or contrasted with the others, or with another); hence, either (of two), or any single individual (of the whole number); this or that: as, from one side of the room to the other.
• Some: used of a single thing indefinitely.
• Single in kind; the same: as, they are all of one age.
• Single; unmarried.
• Certain; some: before the name of a person hitherto not mentioned, or unknown to the speaker. As thus used, one often implies social obscurity or insignificance, and thus conveys more or less contempt.
• Alone; only: following a pronoun and equivalent to self: used reflexively.
• [By a peculiar idiom, the adjective one was formerly used before the article the or an, or a pronoun, followed by an adjective, often in the superlative (as “one the best prince”), where now the pronoun one, followed by of and a plural noun (partitive genitive), would be used (as “one of the best princes”). Compare the idiom in “good my lord,” etc.
• A matter of indifference; of no consequence.
• Completely; entirely; out and out.
• Identical with; the same as.
• noun The first whole number, consisting of a single unit; unity.
• noun The symbol representing one or unity (1, I, or i).
• noun The same.
• A single person or thing; an individual; a person; a thing; somebody; some one; something.
• The most frequent constructions of one are — As antecedent to a relative pronoun, one who being equivalent to any person who, or to he who, she who, without distinction of gender.
• As a substitute for a noun used shortly before, avoiding its repetition: as, here are some apples; will you take one? this portrait is a fine one.
• After an adjective, as substitute for a noun easily supplied in thought, especially being, person, or the like.
• It easily passes, however, from the meaning ‘any one’ into the collective sense of ‘all persons,’ ‘people generally,’ and for this can be substituted people, they, we (if the speaker does not except himself from the general statement), you (the person addressed being taken as an example of others in general), or the impersonal passive may be substituted: as, one cannot be too careful (we cannot, you cannot, they cannot, people cannot be too careful); one knows not when (it is not known when). One is sometimes virtually a substitute for the first person, employed by a speaker who does not wish to put himself prominently forward: as, one does not like to say so, but it is only too true; one tries to do one's best. One's self or oneself is the corresponding reflexive: as, one must not praise one's self.
• [capitalized] A certain being, namely the Deity; God: the name being avoided from motives of reverence or from reserve.
• To make one; unite into a whole; join.
• In chem., a termination of hydrocarbons belonging to the series which has the general formula C_nH_2n-4: as, pentone, C₅H₆.

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

• noun A single unit.
• noun A symbol representing a unit, as 1, or i.
• noun A single person or thing.
• noun [Obs.] after one fashion; alike.
• noun in agreement or concord. See At one, in the Vocab.
• noun [Obs.] continually; perpetually; always.
• noun in union; in a single whole.
• noun singly; one at a time; one after another.