The headword phrase summarises the following quoted text (on which see below), with slight variation in the first of the two terms being contrasted: mo/non is the neuter nominative, vocative, and accusative (and masculine accusative) singular form of the adjective mo/nos, -h, -on ("alone, solitary"; see LSJ s.v. The other term, e(/n ("one"), is the neuter form of the numeral adjective ei(=s and mi/a, masculine and feminine gender, respectively (see LSJ s.v. and Smyth §347).
[1] The importance of the difference between noetic (pure, ideal, intelligible, abstract) and aesthetic (sensible, tangible, concrete) numbers to Greek philosophy and mathematics is explained by Klein (p. 10 and pp. 46-51).
[2] Together with the headword phrase, this quotation approximates the Life of Pythagoras in Photius, Bibliotheca 249.438b33-8. The Suda omits that part of Photius' entry attributing the distinction between noetic and aesthetic numbers to the followers of Pythagoras (C6 BCE); cf. pi 3120, pi 3121, pi 3123, pi 3124, and OCD(4) s.v. But Aristotle, Metaphysics 987a19-20 (web address 1), asserts that the Pythagoreans had no such distinction; they maintained that number was the being (ta\ ou)si/a, "essence") of all things. Indeed he claims that the distinction is Platonic in origin. At Metaphysics 987b7-11 (web address 2) he points to Plato's separation of numbers as Forms or Ideas from sensible things and further to his mentor's having posited a realm of "mathematical entities" intermediate between noetic and aesthetic being; cf. Metaphysics 987b14-18 (web address 2) and Burkert, pp. 30-1.