What does the letter stand for in your mcat test?

There is no "letter" that stands for anything in the MCAT test as a whole. The MCAT (Medical College Admission Test) is a standardized exam, and the letters simply refer to the sections:

* C/P: Chemical and Physical Foundations of Biological Systems

* CARS: Critical Analysis and Reasoning Skills

* BB: Biological and Biochemical Foundations of Living Systems

* PS: Psychological, Social, and Biological Foundations of Behavior

Each section is scored separately, and then a combined score is calculated. There isn't a single letter representing the entire test.