Definition of that :
1. As a conjunction, that retains much of its force as a demonstrative pronoun.
2. As a demonstrative pronoun ( pl. Those), that usually points out, or refers to, a person or thing previously mentioned, or supposed to be understood. That, as a demonstrative, may precede the noun to which it refers; as, that which he has said is true; those in the basket are good apples.
3. As adverb: To such a degree; so; as, he was that frightened he could say nothing.
4. As an adjective, that has the same demonstrative force as the pronoun, but is followed by a noun.
5. In an elliptical sentence to introduce a dependent sentence expressing a wish, or a cause of surprise, indignation, or the like.
6. To introduce a clause employed as the object of the preceding verb, or as the subject or predicate nominative of a verb.
7. To introduce a consequence, result, or effect; - usually preceded by so or such, sometimes by that.
8. To introduce a purpose; - usually followed by may, or might, and frequently preceded by so, in order, to the end, etc.
9. To introduce, a reason or cause; - equivalent to for that, in that, for the reason that, because.