Split knowledge
The requirement that no single person ever knows a whole key, only one component of it.
In payments and around HSMs, a master key is split into two or three components, each held by its own custodian — often in a sealed envelope in a separate safe. At load time the custodians enter their components one by one, and the HSM combines them with XOR. A key check value per component lets each custodian confirm they typed it correctly without revealing anything to the others.
Split knowledge almost always travels with dual control, but they are not the same: dual control requires two people to be present, split knowledge ensures neither of them knows enough to manage without the other. Threshold schemes such as Shamir's secret sharing go further and tolerate a custodian losing their share.