Final answer:
The geoid can be defined as the surface of Earth's gravity fields, representing the irregular shape and size of Earth due to variations in mass distribution. It is not a perfect sphere but rather an oblate spheroid.
Step-by-step explanation:
Geoid can be defined as the surface of Earth's gravity fields. It represents the irregular shape and size of Earth, accounting for variations in gravity caused by variations in mass distribution across the planet. The Earth's geoid shape is not a perfect sphere but instead more closely resembles an oblate spheroid, slightly flattened at the poles and bulging at the equator.