An incident in 2374 in which the USS Voyager detected Omega molecules made by a Delta Quadrant civilisation and, under Starfleet's most secret order, set out to destroy them.
The Omega Directive incident is told in the Voyager episode The Omega Directive. When the ship's sensors pick up the Omega molecule the computers lock, and only Kathryn Janeway may see why. Omega is the most powerful substance known, and a single unstable molecule can ruin subspace across a wide area, as an accident in the Lantaru sector once showed.
The directive tells a captain to set aside every other duty, the Prime Directive included, and destroy the molecules. Seven of Nine already knows about Omega, because the Borg regard it as perfection and once tried to stabilise it.
The source is a research station wrecked by an explosion. Its builders want Omega as a source of energy and object to Voyager taking it. Seven designs a chamber to break the molecules down, and just before they are destroyed she watches them briefly stabilise by themselves.