Verbal Nouns.—Verbal forms that share certain properties of nouns are called
1. Substantival: the infinitive.
N.—The infinitive is properly a case form (chiefly dative, rarely locative), herein being like a substantive.
2. Adjectival (inflected like adjectives):
a. Participles: active, middle, and passive.
b. Verbal adjectives:
In -τός, denoting possibility, as φιλητός
In -τέος, denoting necessity, as γραπτέος
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].