necessary and sufficient
Used in logic, mathematics, and formal contexts to describe conditions that are both required and enough. In everyday language, it can mean 'just right' or 'exactly what is needed'.
This condition is necessary and sufficient.
Please add a necessary and sufficient amount of water.
必要 means 'necessary' but does not imply sufficiency. 必要十分 explicitly adds the idea of being enough.
十分 means 'sufficient' but does not imply necessity. 必要十分 combines both concepts.
Compound of 必要 (necessary) and 十分 (sufficient). The exact historical derivation is uncertain, but it is a straightforward combination of two common Sino-Japanese words.