Sometimes, a reaction may not just occur in one single step. Instead, it may take place over a series of steps. For example, if X, Y and Z were to react, X and Y may first form a reaction intermediate, then that reaction intermediate may then react with Z to form the final products.
Each single step in an overall reaction is called an elementary step, and the series of elementary steps is called a reaction mechanism.
The slowest elementary process is called the rate-determining step, and its rate law is the rate law for the overall reaction.
