adjective

1. being a single thing or unit; not two or more
2. characterized by unity; forming a whole; united; undivided with one accord
3. designating a person or thing as contrasted with or opposed to another or others from one day to another
4. being uniquely or strikingly the person or thing specified the one solution to the problem
5. single in kind; the same all of one mind
6. designating a single, but not clearly specified, person or thing; a certain [one day last week]: also used as an intensive substitute for the indefinite article [she's one beautiful girl]

Etymology: ME < OE an, akin to Ger ein, Goth ains < IE *oinos (> Gr oinē, L unus, OIr ōen) < *e-, *ei-, prefixed pronominal stem meaning “the, this, this one”

noun

1. the number expressing unity or designating a single unit: the lowest cardinal number and the first used in counting a series; 1; I
2. a single person or thing
3. something numbered one or marked with one pip, as the face of a die or domino
4. ☆ Informal a one-dollar bill

pronoun

1. some, or a certain, person or thing one of us must go
2. any person or thing; anybody or anything [when one is exhausted, it is wise to rest]: sometimes used affectedly in place of a personal pronoun in the first or second person what else could one do?
3. the person or thing previously mentioned they rent a house, but I own one