Entities in a Proof of Stake network that contribute an amount of capital to a one or more validators, but don’t actually run the an authoring node or produce blocks, they only play a role in increasing Economic Security. In effect, they are willing to attest that the said validator(s) are honest.
Proof of Stake networks that allow this are called Delegated/Nominated Proof of Stake networks.
By convention, when an entity can just chose one validator to back, it is called Delegation. When multiple parties can be selected, it is called Nomination. Ideally, in Nominated Proof of Stake, an optimization process can split the total stake of a nominator