Operation of law
The phrase by operation of law is a legal term that indicates that a right or liability has been created for a party, irrespective of the intent of that party, because it is dictated by existing legal principles. For example, if a person dies without a will, his heirs are determined by operation of law. Adverse possession, in which title to land passes because non-owners have occupied it for a certain period of time, is another important right that vests by operation of law.
Generally, rights or liabilities created by operation of law are created because a contingency occurs for which a party's has failed to plan (e.g. failure to write a will); or because a specific condition exists for a set period of time (e.g. adverse possession of property or creation of an easement; failure of a court to rule on a motion within a certain period automatically defeating the motion; failure of a party to act on a filed complaint within a certain time causing dismissal of the case); or because an existing legal relationship is invalidated, but the parties to that relationship still require a mechanism to distribute their rights (e.g. under the Uniform Commercial Code, where a contract for which both parties have performed partially is voided, the court will create a new contract based on the performance that has actually been rendered, and containing reasonable terms to accommodate the expectations of the parties).