In Koiné Greek, a
substantive adjective is an adjective that functions as a noun in a sentence and appears in any position other than the predicate position.
A substantive adjective is not a noun in the proper sense of the word. A substantive adjective is an adjective that stands in place of a noun rather than describing a noun. When a substantive adjective is translated into English, usually a noun or pronoun must be supplied. For example, when the adjective αγαθος (“good”) is used as a substantive adjective, it is usually translated into English as “good one” or “good person” or “good thing,” etc.
Matthew 7:11 ὁ πατὴρ ὑμῶν ὁ ἐν τοῖς οὐρανοῖς δώσει ἀγαθὰ τοῖς αἰτοῦσιν αὐτόν