1.pronoun(demonstrative) The thing, person, idea, quality, event, action or time indicated or understood from context, especially if more remote geographically, temporally or mentally than one designated as "this", or if expressing distinction.
"He went home, and after that I never saw him again."
2.pronounThe known (thing); used to refer to something just said.
"They're getting divorced. What do you think about that?"
3.pronoun(demonstrative) The aforementioned quality or proposition; used to emphatically affirm or deny a previous statement or question.
"The water is so cold! — That it is."
4.pronoun(relative) (plural that) Which, who; representing a subject, direct object, indirect object, or object of a preposition.
"The CPR course that she took really came in handy."
5.pronounUsed in place of relative adverbs such as where or when; often omitted.