All possible keys in a specific system. In the RC5-64 project, there are more than 68,000,000,000 blocks with even more keys. 30% of the keyspace has been exhausted, meaning that 30% of the keys have been tested. See also: RC5-64, key.
all possible keys for a particular cryptography system. Typically measured in bit length.
The key space is the complete list of all keys or passwords for a certain job. Since it is usually very huge, it is distributed over the clients, and each of them looks at different pieces of the keyspace. This is what this project is all about, afterall: distributed computing.