What is De Aequitate?

Legal Definition
In equity. De jure stricto, nihil possum vendicare, de oequitate tamen, nullo modo hoc obtinet; In strict law, I can claim nothing, but in equity this by no means obtains. Fleta, 11b. 3, c. 2, § 10.
-- Black's Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary