A discrete semiconductor is basically the opposite of an integrated circuit. It is an individual circuit that serves only one function as a single semiconductor, as opposed to multiple semiconductor components that you might find on a printed circuit board (PCB).
They are generally created for high power or frequency functions and as a result require specialist packaging.
Here are some individual device examples:
As they are the most basic parts of engineering, there are a wide range of uses, in both industrial and consumer applications: