In the Lab Model (also known as CIELab or CIE L*a*b*)
L stands for Lightness.
a stands for red/green chrominance.
b stands for yellow/blue chrominance.
The Lab color model was developed in 1931 by La Commision internationale de L'Eclairage (CIE). It is device independent, and uses synthetic primary colors to more closely model the way the human eye perceives color.