What are physics "codes?" How does the physicist define “code?” I often read slug lines such as “New hydrodynamic codes reveal stellar core collapse” or “Scientists use EMSolve Code to better understand magnetic fusion energy.” To most laymen, code is a set of instructions, i.e. a computer program. However, when I peruse the paper or article the code in question is often a set of mathematical equations. It is certainly not computer code, pseudo-code, or even an algorithm. I am perplexed and hence the question.