Final answer:
A drop in sea level is referred to as regression, a geological process that causes previously submerged seafloor areas to be exposed above sea level.
Step-by-step explanation:
A drop in sea level is called a regression. This geological process occurs when areas of the seafloor that were previously submerged become exposed above sea level. For instance, if we observe a sedimentary sequence showing sediment packages moving in a direction away from the coastline, and these are stacking on top of each other away from the coastline, this pattern is known as offlapping. Such sedimentary patterns can be indicative of a marine regression, as they reveal that the sea level was falling and the coast was once in a different location. Conversely, the geological process that involves areas of land becoming submerged below sea level is known as a transgression, which causes onlapping sedimentary patterns.