1. International law.
Approval of an international treaty by the relevant head of state, or the head of state and the legislature, when necessary to bring the treaty into force.
Most international treaties state expressly whether or not ratification is required, to make them effective.
2. Contract law - agency.
Confirmation or adoption of an act, where necessary for it to have legal effect.
For example, if an agent - without authority - forms a contract with a third party, the principal can ratify and adopt the contract, making it fully effective and enforceable.
3. Company law - sanctioning minor irregularities.
In the case of minor irregularities in running a company, a general meeting can pass a resolution to sanction the irregularity.
Major irregularities cannot be sanctioned in this way, for example ultra vires acts, or a fraud on the minority.
Source: Oxford Dictionary of Law, 8th Edition