Translation guide
The English word "monad" is a technical term from philosophy and computer science. In everyday Japanese, it is almost always expressed using the loanword モナド (monado). There is no common native Japanese equivalent.
Referring to an indivisible, fundamental unit of reality, as in the philosophy of Leibniz.
The standard loanword used in Japanese philosophical discourse for Leibniz's monads or similar concepts.
ライプニッツの哲学では、モナドはこれ以上分割できない実体です。
In Leibniz's philosophy, monads are indivisible substances.
A less common kanji translation, literally 'single/unit child'. Sometimes used in older or more literary philosophical texts.
単子論はライプニッツの主著の一つです。
Monadology is one of Leibniz's major works.
Referring to a monad as a design pattern in functional programming, used to handle side effects, state, etc.
The universally used term in Japanese programming contexts. No native Japanese word is used.
Haskellでは、IOモナドを使って副作用を扱います。
In Haskell, you use the IO monad to handle side effects.
Referring to any single, indivisible unit in a non-specialist context (very rare).
Since the concept is not widely known outside specialist fields, you may need to briefly explain it. The loanword モナド is still the best choice.
このシステムの基本単位は「モナド」と呼ばれるものです。
The basic unit of this system is called a 'monad'.
Do not attempt to translate 'monad' using words like 単位 (unit) or 実体 (substance) unless you are specifically explaining the concept. The loanword モナド is the standard and expected term in both philosophy and computer science.