Definition of primitive type

This page covers the `IsPrimitive` property for types, but I haven't been able to find a definition anywhere of what a primitive type is. As far as I can tell primitive types are the value types built into .Net. Structs and Enums aren't primitive types (but are value types) as they can be user defined.

There was this [discussion](https://stackoverflow.com/questions/2066035/in-c-sharp-are-the-terms-primitive-and-literal-interchangeable/2135136#2135136), that says that 'primitive' isn't defined in the specification, but there must be a reason why the types listed on this page are deemed primitive.

This [site](https://medium.com/omarelgabrys-blog/primitive-data-types-in-c-vs-java-5b8a597eef05) describes primitives as the main built-in types, but `string`, `object`, aren't primitive as per the list on the `IsPrimitive` page, but maybe this definition isn't specific to value types.

Is there a definition, and can it be added to this page, and maybe else where in the msdn documentation?

Here's some more links:
[This page](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/built-in-types-table) mentions simple types, which seems to be the same as primitive types, but there is once again no definition of what a simple type is other than the list of types.
Here's another [page](https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2008/ms228360(v=vs.90)) that talks about primitive types with respect to both C# and Java.

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* Content: [Type.IsPrimitive Property (System)](https://docs.microsoft.com/en-us/dotnet/api/system.type.isprimitive?view=netframework-4.7.2)
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