Round key
The subkey used in a single round of a block cipher, derived from the main key by the key schedule.
A block cipher applies its round function many times, and each round needs its own key material. The key schedule expands the main key into that sequence. AES-128 uses eleven 128-bit round keys, and the main key itself is never fed directly into a round.
For cryptanalysts, round keys are often the better target. If an attacker can pin down a few of them, the rest usually follow, since the schedule is deterministic and in many designs can be run backwards. Many attacks on reduced-round variants therefore guess round-key bits in small groups instead of searching for the main key head-on.