PSA: Prostate Specific Antigen. The blood level rises in diseases of the prostate, including prostate cancer. It's a marker and can be a warning sign. However, it's nonspecific, and a number of things can raise the level, including digital examination of the prostate by a physician and, as I meant to point out, riding a bicycle on a seat that is narrow enough to impinge upon the prostate.
I learned this the hard way--though I had heard it, I was doubtful--when my PSA suddenly shot up, and, after considerable testing and other investigation, we proved that it was due to bicycle riding. (I then got a wide bicycle seat.)