What is the word for something that is non-divisible?

I'm looking for a generic word, an adjective, that means non-divisible. Like the opposite of compound.

I am looking for a word I can use in the context of programming, where you can have data types that are compound of other data types. I want a word to identify the "indivisible" data types.

The best word for something that cannot be split in a programming context is atomic.

This is used a lot, and is essential for interruptible programming. For example, in a signal handler you should only touch variables of type sig_atomic_t and storage class volatile, as explained here.

Quoting from your comment:

I was looking for something in the context of programming where you can have data types that are compound of other data types. I wanted a word to identify the "indivisible" data types.

It sounds like you're asking for a word to describe types like int, double, char, etc. in most programming languages. The most common term I've heard for those is primitive. Some other languages identify them as "built-in" or "basic," but "primitive" is most likely to be recognized by the majority of developers (across all languages) as a term for what you're talking about.

In other contexts, "primitive" would probably not be used to designate an indivisible object. You'd be more likely to hear "indivisible" itself, "fundamental," or "atomic."