In F1 delta is the difference between two lap times.

A slower lap will show a positive delta and a faster lap a negative delta. A driver can compare his own delta over the lap or sectors within the lap and you may hear their engineer giving them their delta to another driver.

Measured in tenths, hundredths and thousandths of a second, the delta is displayed by the broadcaster in yellow for positive (slower) and green for negative (faster). 

Andrew Burden

The author Kiwi F1 Fan