The four elements required to form a valid contract — and which an insurance contract must satisfy — are offer, acceptance, consideration, and legal capacity/legal purpose. Insurance contracts are additionally characterized by four distinctive attributes: Conditional, Unilateral, Adhesion, and Aleatory. The 'aleatory' characteristic refers to the fact that:

Why this is the answer

Insurance contracts share four distinctive characteristics: (1) Conditional — the insurer's duty to pay is triggered only when specified conditions are met (covered loss, proof of loss, premium current); (2) Unilateral — only the insurer makes an enforceable promise after policy issuance; the insured's premium payment is a condition, not a promise; (3) Adhesion — the insurer drafts and the insured 'takes it or leaves it,' so ambiguities are construed against the drafter (contra proferentem); (4) Aleatory — the parties exchange unequal values contingent on a fortuitous event (the insured pays a relatively small premium for the possibility of a large benefit; the insurer collects premiums knowing some insureds will trigger large claims while most will not).