In homological algebra, a monad is a 3-term complex

A → B → C

of objects in some abelian category whose middle term B is projective, whose first map A → B is injective, and whose second map B → C is surjective. Equivalently, a monad is a projective object together with a 3-step filtration B ⊃ ker(B → C) ⊃ im(A → B). In practice A, B, and C are often vector bundles over some space, and there are several minor extra conditions that some authors add to the definition. Monads were introduced by Horrocks (1964, p.698).

• ADHM construction

• Barth, Wolf; Hulek, Klaus (1978), "Monads and moduli of vector bundles", Manuscripta Mathematica, 25 (4): 323–347, doi:10.1007/BF01168047, ISSN 0025-2611, MR 0509589, Zbl 0395.14007
• Horrocks, G. (1964), "Vector bundles on the punctured spectrum of a local ring", Proceedings of the London Mathematical Society, Third Series, 14 (4): 689–713, doi:10.1112/plms/s3-14.4.689, ISSN 0024-6115, MR 0169877, Zbl 0126.16801