In the Hippocratic text Diseases I, there’s actually something of a definition of phrenitis offered (text copied from the Loeb), and it’s a really interesting attempt to describe a mental illness:
72. Phrenitis: something like a thorn seems to be in the inward parts and to prick them; loathing attacks the patient, he flees light and people, he loves the dark, and he is seized by fear. His diaphragm swells outwards, and is painful when touched. The patient is afraid, and he sees terrible things, frightful dreams, and sometimes the dead. This disease attacks most people in spring.
72. Φρενῖτις· δοκεῖ ἐν τοῖσι σπλάγχνοισιν εἶναι | οἷον ἄκανθα καὶ κεντέειν, καὶ ἄση αὐτὸν λάζυται· καὶ τὸ φῶς φεύγει καὶ τοὺς ἀνθρώπους, καὶ τὸ σκότος φιλέει, καὶ φόβος λάζεται. καὶ αἱ φρένες οἰδέουσιν ἐκτός, καὶ ἀλγέει ψαυόμενος. καὶ φοβεῖται, καὶ δείματα ὁρᾷ καὶ ὀνείρατα φοβερὰ καὶ τοὺς τεθνηκότας ἐνίοτε. καὶ ἡ νοῦσος [ἐνίοτε] λαμβάνει τοὺς πλείστους τοῦ ἦρος.