A monad is:
1) A type of functor used in category theory in mathematics. Category theory describes patterns in mathematical functions.
2) A kind of constructor used in input/output (I/O) operations without using language features. In general, however, monads are useful whenever a programmer wants to perform a purely functional computation separate from a related computation performed apart from it.
3) The original code name for Microsoft's Powershell, an object-oriented programming language and interactive command line shell for Microsoft Windows with built-in, flexible scripting abilities.
4) A symbol used by ancient Greek philosophers, including Plato, Pythagoras and Aristotle, to describe God or the totality of all beings. Metaphysical and theological theory describes "monism" as the concept of "one essence."