Is the Mid-Atlantic Ridge a ridge or a trench?
Trench has a specific meaning in plate tectonics. It doesn't just refer to any valley; it specifically refers to the features formed at subduction zones by the flexure of the downgoing oceanic plate. A classic example is the Marianas Trench which is the location of the deepest point in the ocean



Fast spreading ridges do indeed have valleys at their centre but these are not usually referred to as trenches to avoid confusion. In slow spreading ridges the valley is often filled up with erupted lavas. However, the valley sits on top of a much larger ridge, which is the surface expression of hot mantle upwelling below the spreading axis.