That
That is a versatile word in the English language. It functions as a demonstrative determiner and pronoun, a relative pronoun, and a conjunction. As a demonstrative determiner, it precedes a singular noun: that idea, that book. For plural references, the corresponding determiner is those. As a demonstrative pronoun, it can stand for a noun mentioned or understood: That is expensive. As a relative pronoun, it introduces restrictive clauses: the book that I bought. In nonrestrictive relative clauses, which is preferred in British English; in American usage, that is often avoided. As a conjunction, it introduces content clauses: I think that you are right.
Etymology: From Old English þæt, from Proto-Germanic *þat, cognate with Dutch dat and German das.