As for Comedy, it is (as has been observed) an imitation of men worse than the average; worse, however, not as regards any and every sort of fault, but only as regards one particular kind, the Ridiculous, which is a species of the Ugly. The Ridiculous may be defined as a mistake or deformity not productive of pain or harm to others; the mask, for instance, that excites laughter, is something ugly and distorted without causing pain.

[ἡ δὲ κωμῳδία ἐστὶν ὥσπερ εἴπομεν μίμησις φαυλοτέρων μέν, οὐ μέντοι κατὰ πᾶσαν κακίαν, ἀλλὰ τοῦ αἰσχροῦ ἐστι τὸ γελοῖον μόριον. τὸ γὰρ γελοῖόν ἐστιν ἁμάρτημά τι καὶ αἶσχος ἀνώδυνον καὶ οὐ φθαρτικόν, οἷον εὐθὺς τὸ γελοῖον πρόσωπον αἰσχρόν τι καὶ διεστραμμένον ἄνευ ὀδύνης.]

Aristotle (384-322 BC) Greek philosopher

Poetics [Περὶ ποιητικῆς, De Poetica], ch. 5 / 1449a (c. 335 BC) [tr. Bywater (1909)]
    (Source)

Original Greek. Alternate translations:

Comedy is, as we have said, an imitation of characters of a lower type -- not, however, in the full sense of the word bad, the ludicrous being merely a subdivision of the ugly. It consists in some defect or ugliness which is not painful or destructive. To take an obvious example, the comic mask is ugly and distorted, but does not imply pain.
[tr. Butcher (1895)]

Comedy is, as we stated, the portrayal of an inferior class, yet not in all their inferiority, being the ludicrous side of ugliness abstracted. Ludicrousness is the painless and non-destructive variety of the species ugliness of the genus failing; thus, e.g., a ludicrous countenance is ugly and distorted, but not painful.
[tr. Margoliouth (1911)]

Comedy, as we have said, is a representation of inferior people, not indeed in the full sense of the word bad, but the laughable is a species of the base or ugly. It consists in some blunder or ugliness that does not cause pain or disaster, an obvious example being the comic mask which is ugly and distorted but not painful.
[tr. Fyfe (1932), sec. 3.4]

Comedy is, as we said, a representation of people who are rather inferior -- not, however, with respect to every kind of vice, but the laughable is [only] a part of what is ugly. For the laughable is a sort of error and ugliness that is not painful and destructive, just as, evidently, a laughable mask is something ugly and distorted without pain.
[tr. Janko (1987), sec. 2.4]

Comedy is, as we said, a mimesis of inferior persons not however that it has to do with the whole range of wickedness but with what is funny -- an aspect of ugliness. A funny thing, to be precise, is a clumsy mistake that is not painful or destructive: or to take an obvious example, the comic mask is ugly and grotesque but not repulsive or painful.
[tr. Whalley (1997)]

Comedy, as we said, is an imitation of people of a lower sort, though not in respect to every vice; rather, what is ridiculous is part of what is ugly. For the ridiculous is a certain sort of missing the mark and a deformity that is painless and not destructive; an immediate example is the comic mask, which is something deformed and misshapen without causing pain.
[tr. Sachs (2006)]

Comedy is, as we said, representation of people who are inferior but not wholly vicious: the ridiculous is one category of the embarrassing. What is ridiculous is some error embarrassment that is neither painful nor life-threatening; for example, a comic mask is ugly and distorted but does not cause pain.
[tr. Kenny (2013)]