Jamieson, Fausset and Brown on Revelation 20:2

2. that old--ancient serpent (Re 12:9).

thousand years--As seven mystically implies universality, so a thousand implies perfection, whether in good or evil [Aquinas on ch. 11]. Thousand symbolizes that the world is perfectly leavened and pervaded by the divine; since thousand is ten, the number of the world, raised to the third power, three being the number of God [Auberlen]. It may denote literally also a thousand years.