(law) A proprietary interest (such as ownership, a right or some other benefit) in relation to some land, which may be held in gross or appurtenant to some other interest.

One's personal interest or advantage, especially when pursued without regard for others.

(law) An indefeasible right or title, distinguished from a contingent interest, which could be defeated (i.e. cease) if a certain event occurred.

(banking, finance) Interest, as on a loan or a bank account, that is calculated on the total on the principal plus accumulated unpaid interest.

(finance) The total number of contracts outstanding between market participants in a market for futures, options, or other derivatives.

An advocacy group.

Having self-interest; particularly concerned for oneself.

(law enforcement, chiefly Canada, US) Someone who police have reason to believe may have a connection to a crime, or to possess important information relating to that crime, but who has not been accused or charged by authorities nor been officially deemed to be a suspect.

(law) A right, typically in the form of regular income, that lasts for the lifetime of the person benefiting from that right.

(idiomatic) a desire for, or interest in, the success of a particular person or group, often a sports team