The geoid is a model of Earth's shape that represents its mean sea level across the globe. It is an irregular, three-dimensional surface that accounts for variations in gravitational pull caused by factors like mountains and ocean trenches. The geoid helps scientists understand how gravity affects water flow and sea level. In geodesy, the study of Earth's shape and size, the geoid serves as a reference point for measuring elevations and depths. It is essential for accurate mapping and navigation, as it provides a baseline for understanding how land and water interact on our planet.