Martin Escardo on Nostr: Mathematicians: What are the important *definitions* in your field? For example, in ...

Mathematicians: What are the important *definitions* in your field?

For example, in topology the notions of compactness and of Hausdorff space are crucial (among many others).

Or in complexity theory the notion of NP-completeness.

And so on.

Can you list the ones in your mathematical field?

Non-practicing-mathematicians: this time just listen what the practicing mathematicians say.

This is an exercise, for me, in learning which concepts working mathematicians of fields different from mine consider to be important for their own field.

PS. I will consider Theoretical Computer Scientists and Theoretical Physicists for this question. And I am also willing to be more general, without any specific example in mind.