Definition. An amount of funds in an income trust that may be transferred to the unit holders. Can also be used in reference to estate trusts in that it is the maximum taxable amount of income to be distributed from that trust to the receiving beneficiary.n amount of funds in an income trust that may be transferred to the unit holders. Can also be used in reference to estate trusts in that it is the maximum taxable amount of income to be distributed from that trust to the receiving beneficiary.