What is the meaning of Monad?

One thing, one being, one item.

A group of entites or items treated as one entity.

An ultimate atom, or simple, unextended point; something ultimate and indivisible.

A single individual (such as a pollen grain) that is free from others, not united in a group.

A single-celled organism.

A monoid object in the category of endofunctors of a fixed category.

A data type which represents a specific form of computation, along with the operations "return" and "bind".

Source: wiktionary.org