What does the letter p stand for in the p-51 mustang?

Pursuit

"Pursuit" was an old term that dated back to just after World War 1. The letter prefix was used by the Army to designate the role of the airplane. Thus:

B = Bomber

P = Pursuit

A = Attack

C = Cargo

T = Trainer

O = Observation

L = Liason

then later they finally used

F = Fighter

The numbers were assigned to a new aircraft design as they were introduced. Thus the P-40 was an early a/c that was operational at Pearl Harbor attack. Then came the P-47 and the P-51. The "missing" numbers were aircraft that were experimental a/c that never went into production.

The Navy had their own aircraft code designations. Later, the US Air Force(created in 1947) created some new designations and used 2 letters. Army adapted 2-letter codes for their variety of helicopters(H).