An integer variable containing an address in memory, ie "pointing" to it. Some ProgrammingLanguages? even permit you to do Pointer arithmetics.

Fiddling with Pointers is a very error prone task. Therefor, Pointer arithmetic is nowadays frowned upon unless you are doing low level systems programming.

Pointers also break type safety. Many ProgrammingLanguages? attempt to alleviate this by requiring you to use a different type of Pointer for each kind of DataType you want to point to.

Nowadays, if you aren't writing a kernel or hardware driver, you should be using a ProgrammingLanguage that has GarbageCollection and frees you from the arduous task of keeping track of your memory.