What is Accumulation Trust?

Legal Definition
An accumulation trust is a trust in which the trustee does not distribute the income from the trust, but instead gathers the income and any profits from sale of trust assets and holds these in the trust until the trust terminates (at a time specified in the creating document).

These trusts are disfavored, and often limited by law on a state-by-state basis.
Legal Definition
The trust that holds onto income so the capital will increase rather than paying out the income to a beneficiary.