The phrase "sheer drop" refers to a sudden vertical fall or steep cliff. However, there are a few other synonyms that can be used to describe this type of terrain. One alternative phrase is "precipice," which refers to a dangerously steep or overhanging piece of rock or cliff. "Abyss" is also a fitting description, as it denotes a deep and dark void or pit. The phrase "vertical drop" is perhaps the most straightforward synonym for "sheer drop," highlighting the stark vertical nature of the terrain. Finally, "chasm" can be used to describe a deep and narrow gorge or ravine that might contain a sheer drop.