Validation set
Labelled examples held back from training and used to choose settings such as k, so the test set can stay untouched until the very end.
In a real project you would choose k using a separate validation set — a slice of the training emails held back for tuning — and touch the test set only once, at the very end.
See it in action
Try validation set hands-on in the lesson Learning from examples, part of the AI fundamentals course.