Buffer overflow
Writing more data into a memory buffer than it can hold, corrupting adjacent memory and potentially executing attacker-supplied code.
It is one of the oldest vulnerability classes and remains relevant in software written in languages without automatic memory management, particularly embedded devices and network appliances.
Modern operating system mitigations make exploitation considerably harder than it once was, but they raise the bar rather than removing the class.
Where this comes up
Buffer overflow sits inside our penetration testing work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.