A common way to use up today's high gift-/estate-tax exemption before it sunsets. Risk: if the beneficiary spouse predeceases you, or you divorce, the "access" disappears.
Glossary · trusts
Spousal Lifetime Access Trust
Acronym: SLAT
In plain English
An irrevocable trust for the benefit of your spouse; the assets leave your estate but your spouse retains access, so the family keeps indirect use of the funds.