The shipboard alert condition sounded aboard the USS Discovery whenever the experimental spore drive is about to be engaged for a jump through the mycelial network.
Black alert is unique to starships fitted with a displacement-activated spore hub drive. When it is called, lighting dims, panels show a black alert graphic, and the crew brace as the ship's saucer rings spin and the vessel drops out of normal space.
The condition is first heard in the Discovery episode Context Is for Kings, when the newly arrived Michael Burnham has no idea what it means and sees strange effects ripple through her quarters. It soon becomes routine, called by Captain Lorca during the Klingon war and by his successors afterwards.
The term carries into the 32nd century, where Discovery remains the only ship able to jump, and Burnham as captain gives the order throughout the later seasons.