which acquire new sense in combination

Second, as new concepts are developed, new terms are needed to describe them, so instead of creating new words, we simply put together already existing words, which acquire a new sense in combination.

I fixed it up there for you, Boss. The locution "are needed for their indication" was god-awful lame, and not something a native speaker would have written. If you're set on using the verb to indicate, then write: ". . . new terms are needed to indicate them . . ."