also referred to as indeterminacy or chance music. Aleatory compositions contain at least one aspect (rhythm, pitch, dynamics, etc.) that is left up to the performer to control. The composer usually presents some kind of basic guidelines but it is then up to the performer to "improvise" on these rules.

dependent on chance; "the aleatory element in life"

Improvisation or random in character.

An interpretation of probability concerned with how likely events are to happen. cf.  epistemic

Depending on some uncertain contingency; as, an aleatory contract.

A characteristic of insurance contracts, meaning a contract in which equal value is not given by both parties to the contract.