Simulating and testing the design. You generally don't want to spend millions of dollars on a semiconductor mask set unless you're sure the design will work correctly.

This part usually takes longer than the actual logic design and transistor layout phases.

I don't know the semi-conductor specifics... but it's a broad engineering concept.

Validation: Does the thing you designed actually do what it's supposed to do?

And the related concept,

Verification: Is the thing you build (and deliver) actually what you designed?