At that

(used to indicate a person, thing, idea, state, event, time, remark, etc., as pointed out or present, mentioned before, supposed to be understood, or by way of emphasis):
that is her mother. after that we saw each other.
(used to indicate one of two or more persons, things, etc., already mentioned, referring to the one more remote in place, time, or thought; opposed to this):
this is my sister and that’s my cousin.
(used to indicate one of two or more persons, things, etc., already mentioned, implying a contrast or contradistinction; opposed to this):
this suit fits better than that.
(used as the subject or object of a relative clause, especially one defining or restricting the antecedent, sometimes replaceable by who, whom, or which):
the horse that he bought.
(used as the object of a preposition, with the preposition standing at the end of a relative clause):
the farm that i spoke of.
(used in various special or elliptical constructions):
fool that he is.
(used to indicate a person, place, thing, or degree as indicated, mentioned before, present, or as well-known or characteristic):
that woman is her mother. those little mannerisms of hers make me sick.
(used to indicate the more remote in time, place, or thought of two persons, things, etc., already mentioned; opposed to this):
this room is his and that one is mine.
(used to imply mere contradistinction; opposed to this):
not this house, but that one.
(used with adjectives and adverbs of quant-ty or extent) to the extent or degree indicated:
that much; the fish was that big.
to a great extent or degree; very:
it’s not that important.
dialect. (used to modify an adjective or another adverb) to such an extent:
he was that weak he could hardly stand.
(used to introduce a subordinate clause as the subject or object of the princ-p-l verb or as the necessary complement to a statement made, or a clause expressing cause or reason, purpose or aim, result or consequence, etc.):
i’m sure that you’ll like it. that he will come is certain. hold it up so that everyone can see it.
(used elliptically to introduce an exclamation expressing desire, a wish, surprise, indignation, or other strong feeling):
oh, that i had never been born!
at that,

in spite of something; nevertheless:
although perhaps too elaborate, it seemed like a good plan at that.
in addition; besides:
it was a long wait, and an exasperating one at that.

that is, (by way of explanation, clarification, or an example); more accurately: i read the book, that is, i read most of it.
i believe his account of the story, that is to say, i have no reason to doubt it.
also, that is to say.
that’s that, informal. there is no more to be said or done; that is finished:
i’m not going, and that’s that!
that way, informal. in love or very fond of (usually followed by about or for):
the star and the director are that way. i’m that way about coffee.
with that, following that; thereupon:
with that, he turned on his heel and fled.
determiner (used before a sing noun)

used preceding a noun that has been mentioned at some time or is understood: that idea of yours
(as pr-noun): don’t eat that, that’s what i mean

used preceding a noun that denotes something more remote or removed: that dress is cheaper than this one, that building over there is for sale
(as pr-noun): that is john and this is his wife, give me that compare this

used to refer to something that is familiar: that old chap from across the street
(informal) and that, and all that, everything connected with the subject mentioned: he knows a lot about building and that
(completive-intensive) at that, additionally, all things considered, or nevertheless: he’s a pleasant fellow at that, i might decide to go at that
like that

with ease; effortlessly: he gave me the answer just like that
of such a nature, character, etc: he paid for all our tickets — he’s like that

that is

to be precise
in other words
for example

that’s more like it, that is better, an improvement, etc
that’s that, there is no more to be done, discussed, etc
with that, at that, thereupon; having said or done that
conjunction (subordinating)
used to introduce a noun clause: i believe that you’ll come
also so that, in order that. used to introduce a clause of purpose: they fought that others might have peace
used to introduce a clause of result: he laughed so hard that he cried
used to introduce a clause after an understood sentence expressing desire, indignation, or amazement: oh, that i had never lived!
adverb
used with adjectives or adverbs to reinforce the specification of a precise degree already mentioned: go just that fast and you should be safe
(usually used with a negative) (informal) also all that. (intensifier): he wasn’t that upset at the news
(dialect) (intensifier): the cat was that weak after the fight
pr-noun
used to introduce a restrictive relative clause: the book that we want
used to introduce a clause with the verb to be to emphasize the extent to which the preceding noun is applicable: genius that she is, she outwitted the computer
pr-n.

old english þæt, neuter singular of the demonstrative pr-noun and adjective (corresponding to masc. se, fem. seo), from proto-germanic -that, from pie -tod-, extended form of demonstrative pr-nomial base -to- (cf. sanskrit ta-, lithuanian and old church slavonic to, greek to “the,” latin talis “such”). cf. the.

emerged c.1200 as a demonstrative adjective with the breakdown of the old english grammatical gender system, perhaps by influence of french and latin, which had demonstrative adjectives (old english did not). slang that way “in love” first recorded 1929. that-a-way is recorded from 1839. “take that!” said while delivering a blow, is recorded from early 15c.
in addition, besides, as in the seats were good, and quite cheap at that. [ first half of 1800s ]
in spite of, nevertheless, as in although i had to wait a long time for delivery, it was worth it at that. [ mid-1800s ]
as it stands, without further changes, as in she wasn’t happy with her grade in the course but decided to leave it at that. [ late 1800s ]

that ain’t hay
that does it
that is
that makes two of us
that will do

also see:

all’s well that ends well
all that
all that glitters is not gold
and all (that)
as far as that goes
at that point
at this (that) rate
at this (that) stage
be that as it may
bite the hand that feeds you
cross a (that) bridge
for that matter
game that two can play
how about that
how does that grab you
how’s that
in order (that)
in that
is that a fact
it (that) figures
just like that
just the (that’s the) ticket
last straw (that breaks)
like that
look like the cat that ate the canary
not all that
not built that way
now that
on condition that
on the chance (that)
powers that be
put that in your pipe
seeing that
ships that p-ss in the night
so that
suffice it to say that
tear (that tears) it
this and that
to that effect
when it comes to (that)
would that
you can say that again