A more thorough explanation:

Definition: Interim dominus (in-tər-im dom-ə-nəs) is a term used in Law Latin that refers to a proprietor in the meantime.

For example, a widow is interim domina of terce lands after her service, and in virtue thereof may either possess them herself, or let them out to tenants.

This means that if a widow's husband dies and she is entitled to a portion of his land, she becomes the interim dominus of that land until it is passed on to someone else. During this time, she has the right to either use the land herself or rent it out to others.