Definition - What does Irrevocable Trust mean?
An irrevocable trust is a trust that cannot be altered without the beneficiary's permission after it has become official. In other words, after the grantor has given up the rights to the assets in the irrevocable trust, he or she cannot modify the trust without the consent of the beneficiary.
Justipedia explains Irrevocable Trust
An example of an irrevocable trust would be a landowner who gives a 20 acre piece of land to his nephew in a trust. Since the trust is irrevocable, the nephew (who is the beneficiary), will be the only party involved who has the rights to modify the trust. Grantors in irrevocable trusts essentially give up all control of assets in irrevocable trusts. So in this case, the 20 acre plot would no longer belong to the landowner, and he would would have no say in its future use. This also means that he would be exempt from paying taxes on it.