one (adj., pron., n.)

"being but a single unit or individual; being a single person, thing, etc. of the class mentioned;" as a pronoun, "a single person or thing, an individual, somebody;" as a noun, "the first or lowest of the cardinal numerals; single in kind, the same; the first whole number, consisting of a single unit; unity; the symbol representing one or unity;" c. 1200, from Old English an (adjective, pronoun, noun) "one," from Proto-Germanic *ainaz (source also of Old Norse einn, Danish een, Old Frisian an, Dutch een, German ein, Gothic ains), from PIE root *oi-no- "one, unique."

Originally pronounced as it still is in only, atone, alone, and in dialectal good 'un, young 'un, etc.; the now-standard pronunciation "wun" began c. 14c. in southwest and west England (Tyndale, a Gloucester man, spells it won in his Bible translation), and it began to be general 18c. Its use as indefinite pronoun was influenced by unrelated French on and Latin homo.

Before the name of a person, indicating "hitherto unknown" or not known to the speaker.

One and only "sweetheart" is from 1906. Slang one-arm bandit for a type of slot machine is recorded by 1938. One-night stand is 1880 in performance sense; 1963 in sexual sense. One of the boys "ordinary amiable fellow" is from 1893. One-track mind "mind capable of only one line of thought or action" is by 1915. Drinking expression one for the road is from 1950 (as a song title). One-man band is by 1909 in a literal sense, 1914 figurative. One of those things "unpredictable occurrence" is from 1934 (Cole Porter's song is from 1935).

The conscience clause is one of the weaknesses of the Bill. It is one of those things which tend to create the bitterness. The conscience clause is one of those things which are inseparable from a Bill like this. It is one of those things which divides the sheep from the goats—members can pick them out for themselves—in the playground, in the school. ["Religious Exercises in School Bills," New Zealand Parliamentary Debates, Aug. 13, 1926]

Definitions of one
one (adj.)
of the same kind or quality;
two animals of one species
one (adj.)
having the indivisible character of a unit;
spoke with one voice
Synonyms: unitary
one (adj.)
(informal) very; used informally as an intensifier;
that is one fine dog
Synonyms: right
one (adj.)
indefinite in time or position;
he will come one day
one place or another
one (adj.)
eminent beyond or above comparison;
she's one girl in a million
the one and only Muhammad Ali
Synonyms: matchless / nonpareil / one and only / peerless / unmatched / unmatchable / unrivaled / unrivalled
one (adj.)
used of a single unit or thing; not two or more;
Synonyms: " / i / ane
one (adj.)
being a single entity made by combining separate components;
three chemicals combining into one solution
one (n.)
the smallest whole number or a numeral representing this number;
he has the one but will need a two and three to go with it
they had lunch at one
Synonyms: " / i / ace / single / unity
one (n.)
a single person or thing;
he is the best one
this is the one I ordered