Sector
Each lap is divided into three timed sectors, so a driver's pace can be compared piece by piece rather than only as a whole lap.
Timing screens show them in colour: purple for the fastest anyone has set, green for a driver's own personal best, yellow for slower than both. It is how you can tell mid-lap whether a flying lap is alive.
A "theoretical best" lap combines a driver's fastest three sectors even though they were set on different laps — which is why it is almost always quicker than anything actually driven.