The kind of operation where you use a thing to construct itself. Examples include using a compiler to compile a newer version of itself, using the core of an operating system to "boot" up the rest of the operating system.

In the case of the N Notation the notation itself is used to define its own structure. It does this by using a subset of its structures to allow the definition of more complex structures, which in turn are able to allow the definition of even more complex features and so on.

Another example is the language hierarchy that builds progressively more complex languages on the foundations of previous languages. Or not in the case of C# ;-)

Other terms: recursion, induction, ...