Usability
Usability measures how easily a visitor completes a task on a site: finding information, filling a form, booking a slot.
Usability evaluates whether a site lets someone reach a goal quickly and without error. It's tested through observation: does a visitor find the contact button in a few seconds?
A usable site reduces drop-off. Every unnecessary click, every unclear form field costs leads, especially on mobile where the margin for error is small.