Key space
The set of all possible keys, and the number that decides whether brute force is a theory or a practical plan.
The key space is every key a cipher can use. A Caesar cipher on the Latin alphabet has 25 meaningful keys, which a person can try over an afternoon and a computer in a fraction of a second. DES had 256 keys, about 7.2 × 1016, and in 1998 the EFF's Deep Crack machine searched the entire space in 56 hours.
Size alone is not security. A large key space can still be weak if keys are chosen badly or the cipher has structural flaws, and a small one is a clear sign that a cipher is obsolete. Kerckhoffs's principle makes the key the only secret that matters, which is why the key space is central: it is what the attacker actually has to search.