Monad (functional programming)
A monad is an abstract concept that expresses sequential composition. Basically, it describes the ability to order commands to a sequence, such that later commands can use the results of earlier commands.
A monad is a way to structure computations in terms of values and sequences of computations using those values. Monads allow the programmer to build up computations using sequential building blocks, which can themselves be sequences of computations. The monad determines how combined computations form a new computation and frees the programmer from having to code the combination manually each time it is required.