To appreciate the usage of "monad" as a mathematical term it helps to go back nearly two millennia to the origin of this term in algebra: Diophantus of Alexandria used monad (denoted $\dot{M}$) in his book Arithmetika to indicate the zeroth power of the unknown variable. "All numbers consist of a certain multitude of monads." Think of "monad" as "unit-one". In a later development, Archimedes used monads as building blocks for larger units , for example "myriad" was the "unit-ten-thousand" and "chiliad" the "unit-thousand".

It would seem the modern usages of "monad" follow this ancient line of thought, to indicate "a single entity that generates all other entities".

To appreciate the usage of "monad" as a concept in mathematics (rather than philosophy), it might help to go back nearly two millennia to the first use of this term in algebra [*]: Diophantus of Alexandria used monad (denoted $\dot{M}$) in his book Arithmetika to indicate the zeroth power of the unknown variable. "All numbers consist of a certain multitude of monads." Think of "monad" as "unit-one". In a later development, Archimedes and Apollonius used monads as building blocks for larger units , for example "myriad" was the "unit-ten-thousand" and "chiliad" the "unit-thousand". Archimedes called these "numbers formed analogously to those based on monads". In this way large numbers could be manipulated with ease.

It would seem the modern usage of "monad" follows this ancient line of thought, to indicate "a single entity that generates all other entities".

[*] Jacob Klein, Greek Mathematical Thought and the Origin of Algebra (Dover, 1992).