Held-out set
Also: test set
Data set aside and not looked at during development, used once to estimate how the system will do on inputs it has never seen.
What people get wrong
It stops being held out the moment you tune against it. Every decision informed by that score leaks a little of it into the model, which is what the separate validation set is for.
A definition is not an understanding. This is taught properly in Memorising instead of learning, part of Overfitting & regularisation.
Learn this properly