An insurer can only price a risk it can predict. An ideally insurable risk has six features:
The loss is due to chance: accidental, not intentional, and not certain to happen.
The loss is definite and measurable in money.
The loss is statistically predictable.
The loss is not catastrophic: one event must not be able to bankrupt the insurer.
There is a large number of similar exposure units, so the law of large numbers can work.
Coverage is not mandatory for the insurer: the insurer may choose which applicants to accept.
California's insurable events rule (Ins. Code §250): any contingent or unknown event, whether past or future, that may damage a person who has an insurable interest, or create a liability against that person, may be insured against, subject to the Code.