BSA is bovine serum albumin.
Bovine = cow
Serum = the leftover stuff in your blood that isn't cell-based
Albumin = an important protein in your serum that allows for transport of all sorts of things
We use BSA in science to combat non-specific binding, a very evil thing indeed when you're trying to find out what the affinity of one thing is to another (also know as it's "kd" =name of blog relevance alert!)
So, we often add it in specific assays and such to help us reduce non-specific binding. But then one of the members of our lab doing work with a tetracycline-binding construct were having these really confusing readings...and then we found out that stocks of BSA are contaminated with tetracycline. Tetracycline is an antibiotic. BSA is from cows that are killed at slaughter that are pumped full of antibiotics in order to keep down infection in the very close living quarters of commercial agriculture. Thus---contaminated BSA meant that we had no way to control for how much tetracyline was in these assays. Hence---low cost agriculture completely ruined a few weeks of experiments. Thanks, guys. Come on...let the cows run free and sans antibiotics! Please?