Allen and Greenough's New Latin Grammar, section 156

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SIGNIFICATION OF THE FORMS OF THE VERB.

Voices.

156.

The Active and Passive Voices in Latin generally correspond to the active and passive in English; but -

a. The passive voice often has a reflexive meaning: -

ferr=o accingor, I gird myself with my sword.
Turnus vertitur, Turnus turns (himself).
induitur vestem, he puts on his (own) clothes.

NOTE: This use corresponds very nearly to the Greek Middle voice, and is doubtless a survival of the original meaning of the passive (p. 76, footnote 2).

b. Many verbs are passive in form, but active or reflexive in meaning. These are called Deponents (§ 190):[1][That is, verbs which have laid aside (d=ep=onere) the passive meaning.] as, hortor, I exhort; sequor, I follow.

c. Some verbs with active meaning have the passive form in the perfect tenses; these are called Semi-Deponents: as, audeó, audére, ausus sum, dare.