At one

being or amounting to a single unit or individual or entire thing, item, or object rather than two or more; a single:
one woman; one nation; one piece of cake.
being a person, thing, or individual instance or member of a number, kind, group, or category indicated:
one member of the party.
existing, acting, or considered as a single unit, entity, or individual.
of the same or having a single kind, nature, or condition:
We belong to one team; We are of one resolve.
noting some indefinite day or time in the future:
You will see him one day.
a certain (often used in naming a person otherwise unknown or undescribed):
One John Smith was chosen.
being a particular, unique, or only individual, item, or unit:
I’m looking for the one adviser I can trust.
noting some indefinite day or time in the past:
We all had dinner together one evening last week.
of no consequence as to the character, outcome, etc.; the same:
It’s all one to me whether they go or not.
the first and lowest whole number, being a cardinal number; unity.
a symbol of this number, as 1 or I.
a single person or thing:
If only problems would come one at a time!
a die face or a domino face having one pip.
a one-dollar bill:
to change a five-dollar bill for five ones.
(initial capital letter) Neoplatonism. the ultimate reality, seen as a central source of being by whose emanations all entities, spiritual and corporeal, have their existence, the corporeal ones containing the fewest of the emanations.
a person or thing of a number or kind indicated or understood:
one of the Elizabethan poets.
(in certain pronominal combinations) a person unless definitely specified otherwise:
every one.
(with a defining clause or other qualifying words) a person or a personified being or agency:
the evil one; the one I love.
any person indefinitely; anyone:
as good as one would desire.
Chiefly British. (used as a substitute for the pronoun I):
Mother had been ailing for many months, and one should have realized it.
a person of the speaker’s kind; such as the speaker himself or herself:
to press one’s own claims.
something or someone of the kind just mentioned:
The portraits are fine ones. Your teachers this semester seem to be good ones.
something available or referred to, especially in the immediate area:
Here, take one—they’re delicious. The bar is open, so have one on me!
at one,

in a state of agreement; of one opinion.
united in thought or feeling; attuned:
He felt at one with his Creator.

one and all, everyone:
They came, one and all, to welcome him home.
one by one, singly and successively:
One by one the children married and moved away.
one for the road. (def 10).
determiner

single; lone; not two or more: one car
(as pronoun): one is enough for now, one at a time
(in combination): one-eyed, one-legged

distinct from all others; only; unique: one girl in a million
(as pronoun): one of a kind

a specified (person, item, etc) as distinct from another or others of its kind: raise one hand and then the other
(as pronoun): which one is correct?

a certain, indefinite, or unspecified (time); some: one day you’ll be sorry
(informal) an emphatic word for a1 , an1 it was one hell of a fight
a certain (person): one Miss Jones was named
in one, all in one, combined; united
all one

all the same
of no consequence: it’s all one to me

(often foll by with) at one, in a state of agreement or harmony
be made one, (of a man and a woman) to become married
many a one, many people
neither one thing nor the other, indefinite, undecided, or mixed
never a one, none
one and all, everyone, without exception
one by one, one at a time; individually
one or two, a few
one way and another, on balance
(informal) off on one, exhibiting bad temper; ranting
one with another, on average
pronoun
an indefinite person regarded as typical of every person: one can’t say any more than that
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
(archaic) an unspecified person: one came to him
noun
the smallest whole number and the first cardinal number; unity See also number (sense 1)
a numeral (1, I, i, etc) representing this number
(informal) a joke or story (esp in the one about)
(music) the numeral 1 used as the lower figure in a time signature to indicate that the beat is measured in semibreves
something representing, represented by, or consisting of one unit
Also called one o’clock. one hour after noon or midnight
a blow or setback (esp in the phrase one in the eye for)
the one, (in Neo-Platonic philosophy) the ultimate being
the Holy One, the One above, God
the Evil One, Satan; the devil
n.

c.1200, from Old English an (adjective, pronoun, noun) “one,” from Proto-Germanic *ainaz (cf. Old Norse einn, Danish een, Old Frisian an, Dutch een, German ein, Gothic ains), from PIE *oi-no- “one, unique” (cf. Greek oinos “ace (on dice);” Latin unus “one;” Old Persian aivam; Old Church Slavonic -inu, ino-; Lithuanian vienas; Old Irish oin; Breton un “one”).

Originally pronounced as it still is in only, 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. Use as indefinite pronoun influenced by unrelated French on and Latin homo.

One and only “sweetheart” is from 1906. One of those things “unpredictable occurrence” is from 1934. Slang one-arm bandit “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 is from 1927. Drinking expression one for the road is from 1950 (as a song title).

Related Terms

dead one, fast one, four-and-one, fresh one, hang one on, hot one, number-one boy, square one, thin one

Related Terms

murder one, number one
In agreement, in harmony, as in John and Pat were at one on every subject except her cat, which made him sneeze, or Springtime always makes me feel at one with nature. [ 1300s ]

one and all
one and only
one and the same
one another
one by one
one eye on
one fell swoop, in
one foot in the grave, have
one for the books
one for the road
one good turn deserves another
one in a million
one jump ahead
one man’s meat is another man’s poison
one of a kind
one of these days
one of those days
one on one
one on, that’s
one picture is worth a thousand words
one up
one way or another

also see:

all in one piece
all the same (one)
A-1 (A-one)
as one
at one
at one stroke
at one time
at one time or another
back to the drawing board (square one)
each and every (last one)
each other (one another)
fast one
for one, 1
go one better
hang (one) on
harp on (one string)
hole in one
in one ear and out the other
in the same (in one) breath
irons in the fire, more than one
it takes one to know one
just one of those things
look out for (number one)
more than one way to skin a cat
not one iota
number one
on the one hand
(one) picture is worth a thousand words
put all one’s eggs in one basket
quick one
seen one, seen them all
six of one, half dozen of the other
that’s one on me
tie one on
wear another (more than one) hat
with one arm tied behind one’s back
with one voice
words of one syllable (Note that this listing does not include those idioms where one is a personal pronoun meaning “someone” or “oneself.”)