Impact alert

From Memory Alpha, the free Star Trek reference

You may also be looking for the informal verbal alert "Brace for impact" used commonly in the 24th century.

An impact alert is an alert to notify crewmembers of a starship when it is about to collide with another spatial object, like another vessel.

The viewscreen of the USS Kelvin notified Captain George Kirk of an impact alert with the Nerada after Kirk had set the Kelvin on a collision course with the enemy vessel. Twenty five years later, an impact alert was declared on the bridge of the Enterprise when it warped right into a debris field of its fleet. (Star Trek)