If a key space is the term given to the collection of all possible decryption keys for a given cryptosystem, then what is the collection of all possible encryption keys called?
The key space is the set of all possible keys that you may choose for both encryption and decryption keys. In the case of symmetric key algorithms, both encryption and decryption keys are the same. However, in asymmetric-key algorithms, with one key for encryption and another for decryption, we refer as the key space for both scenarios. I hope that this explanaton will help you.