What is Fleta?

Legal Definition
Fleta is a treatise, written in Latin, with the sub-title seu Commentarius juris Anglicani, on the common law of England. The anonymous author of the book is sometimes referred to as "Fleta", although this is not in fact a person's name. The book acquired its common title because its preface contains a remark that it could be called "Fleta" as it was written in "Fleta": however, the meaning of this comment is unclear (see Authorship below).

From internal evidence, the work appears to have been written in the reign of Edward I, and to have been completed shortly after the year 1290.
-- Wikipedia
Legal Definition
The title of an ancient law book, supposed to have been written by a judge who was confined in the Fleet prison. It is written in Latin, and is divided into six books. The author lived in the reigns of Ed. II. and Ed. III. See lib. 2, cap. 66, § Item quod nullus; lib. 1, cap. 20, § qui coeperunt, pref. to 10th Rep. Edward II. was crowned, A. D. 1306. Edward III. was crowned 1326, and reigned till A. D. 1377. During this period the English law was greatly improved, and the lawyers and judges were very learned. Hale's Hist. C. L. 173. Blackstone 4 Com. 427, says, of this work, "that it was for the most part law, until the alteration of tenures took place." The same remark he applies to Britton and Hingham.
-- Bouviers Law Dictionary
Legal Definition
The name given to an ancient treatise on the laws of England, founded mainly upon the writings of Bracton and Glanville, and supposed to have been written in the time of Edw. I. The author is unknown, but it is surmised that he was a judge or learned lawyer who was at that time confined in the Fleet prison, whence the name of the book.
-- Black's Law Dictionary
Legal Definition
An ancient law treatise believed to have been written about 1300 by a judge while imprisoned in the Fleet.
-- Ballentine's Law Dictionary