Herbert Weir Smyth, A Greek Grammar for Colleges

a. The middle usually denotes that the subject acts on himself or for himself, as λούομαι wash myself, ἀμύ_νομαι defend myself (lit. ward off for myself).

b. The passive borrows all its forms, except the future and aorist, from the middle.

c. Deponent verbs have an active meaning but only middle (or middle and passive) forms. If its aorist has the middle form, a deponent is called a middle deponent (χαρίζομαι gratify, ἐχαρισάμην); if its aorist has the passive form, a deponent is called a passive deponent (ἐνθυ_μέομαι reflect on, ἐνεθυ_μήθην). Deponents usually prefer the passive to the middle forms of the aorist.