Which of the following best defines "irrevocable trust"?

An irrevocable trust is best defined as a trust that prevents the trustor from changing its terms after it has been established. This characteristic is significant because once the trust is created and assets are transferred into it, the trustor relinquishes control over those assets. The trust terms, including provisions related to beneficiaries and distributions, are set and cannot be modified without the consent of all beneficiaries or by court order.

This permanence provides certain advantages, such as potential tax benefits and protection from creditors, since the assets in an irrevocable trust are generally not considered part of the trustor's estate. Additionally, because the trustor cannot revoke or alter the trust unilaterally, it offers a level of security for beneficiaries that the trust will be administered according to the initial intent of the trustor.

The other options describe different types of trusts or characteristics that do not accurately represent what an irrevocable trust is. For instance, a trust that can be altered or revoked at any time describes a revocable trust, not an irrevocable one. Therefore, the defining feature of preventing change once established is crucial in understanding irrevocable trusts.