Short answer. An aleatory contract is one where a party, or both parties, bind themselves to give or do something in return for what the other gives or does, but only upon an uncertain event or one that will happen at an indeterminate time — which is exactly the structure insurance and gambling arrangements share.
What the law says
By an aleatory contract, one of the parties or both reciprocally bind themselves to give or to do something in consideration of what the other shall give or do upon the happening of an event which is uncertain, or which is to occur at an indeterminate time.
Civil Code, Article 2010 — Aleatory Contract Defined. Read the full provision →
The defining feature is the uncertain trigger
What sets an aleatory contract apart from an ordinary one is that performance turns on the happening of an event which is uncertain, or which is to occur at an indeterminate time. In an ordinary contract, both sides generally know from the start what they are getting; in an aleatory contract, at least one side's obligation depends on something that may or may not happen, or that will happen but nobody can say exactly when.
The obligation can run one way or both ways
The definition covers both shapes of this arrangement: one of the parties or both may bind themselves this way. Sometimes only one party's performance is contingent on the uncertain event while the other's is fixed; in other arrangements, both sides' obligations are reciprocally tied to that same uncertain or indeterminate occurrence, so neither side knows in advance exactly what they will end up owing or receiving.
Why insurance fits this description
An insurance arrangement fits this structure closely: the policyholder pays premiums, and the insurer's obligation to give or to do something — paying out on the policy — depends entirely on the happening of an event which is uncertain, such as an accident, illness, or loss, or an event certain to happen but at an indeterminate time, such as death. Neither side knows in advance whether, or exactly when, the insurer's obligation will actually be triggered.
Why gambling shares the same underlying structure
A wager between two parties follows the identical pattern: each side agrees to give something to the other upon the happening of an event which is uncertain — the outcome of the game, race, or contest being wagered on. Both insurance and gambling place the parties' mutual obligations at the mercy of an event neither side controls or can predict with certainty, which is precisely what the definition of an aleatory contract describes, whatever moral or regulatory differences otherwise separate the two.