A trust that can't be modified or terminated without the permission of the beneficiary. The grantor, having transferred assets into the trust, effectively removes all of his or her rights of ownership to the assets and the trust.

This is the opposite of a "revocable trust", which allows the grantor to modify the trust.

The main reason for setting up an irrevocable trust is for estate and tax considerations. The benefit of this type of trust for estate assets is that it removes all incidents of ownership, effectively removing the trust's assets from the grantor's taxable estate. The grantor is also relieved of the tax liability on the income generated by the assets. While the tax rules will vary between jurisdictions, in most cases, the grantor can't receive these benefits if he or she is the trustee of the trust.  

The assets held in the trust can include, but are not limited to, a business, investment assets, cash and life insurance policies.