What is prefix word for necessary?

There isn't a single prefix word for "necessary." However, you can use prefixes to create related words that convey a similar meaning:

* Indispensable: This word emphasizes the essential nature of something, highlighting its lack of replacement.

* Essential: This word focuses on the core importance of something, its fundamental role.

* Requisite: This word signifies something that is required or demanded for a particular purpose.

* Mandatory: This word highlights the compulsory nature of something, implying a requirement or obligation.

While these words don't have a prefix for "necessary" itself, they all express variations of the concept. You can choose the word that best fits your context and the specific nuance you want to convey.