Isolytic weapons are subspace weapons that tear holes in the fabric of subspace, a class of armament banned under the Second Khitomer Accords.
An isolytic burst damages subspace itself, creating a tear that spreads and consumes anything nearby. The Federation and other powers outlawed such weapons because of the lasting harm they cause to space.
The Son'a fired an isolytic burst at the Enterprise-E in the Briar Patch during their attempt to take the Ba'ku homeworld. The subspace tear threatened to engulf the ship, and the crew sealed it by ejecting and detonating the warp core.