The size of the set of all possible keys to a code.

For someone who is into RSA Cryptography, the size of a keyspace is the number of prime numbers that can be expressed in n bits. This is usually very large, which is good.

For people who choose passwords that suck, the size of a keyspace is equal to the number of entries in a Dictionary, plus the number of different First names in their country of origin. While this is very large to a mere mortal, the entire keyspace can be searched in less than a week these days with a mediocre computer.