The Definitions (Ancient Greek: Ὅροι Horoi; Latin: Definitiones[1]) is a dictionary of 184 philosophical terms sometimes included in the corpus of Plato's works.

Given the sophistication of Plato's and Aristotle's efforts in the area of definition, this collection seems to be an elementary text produced by second-rate philosophical study.

[citation needed] Its early date, however, does give it some importance as a source for the history of ancient Platonism.

Definitions 21–107, the main section of the collection, contain concepts from ethics (affects and virtues), political theory, logic, grammar, and epistemology.

Definitions 108–184 are a final appendix that contains a mixture of concepts which sometimes duplicate earlier terms and therefore was probably added at a later date.

[4] Methodologically, Definitions is related to the Platonic Method of Division (diairesis) that progresses from the more general to the more specific, i.e., from 'above' to 'below.'

The anonymous Prolegomena to Platonic Philosophy, which is dated to late antiquity, designates Speusippus as the author.

This edition was the basis for the Latin translation that the humanist Willibald Pirckheimer brought out in Nuremberg in 1523 with the printer Friedrich Peypus.