Attack Vector

An attack vector is the specific way an attacker actually gets in, such as a scam email, a weak password or an out-of-date program with a known flaw. If your attack surface is every possible way into your systems, a vector is the one way a particular attacker chooses to walk through.

It is the difference between knowing your house has ten windows and knowing the burglar climbed in through the unlatched one by the back step. Naming the most likely ways in for your setup is what lets you spend your effort on the defences that actually stop them, rather than guarding everything equally.