This seemingly simple question can be answered in many ways. Here we look at three different pieces that help to define sea level. Shorelines are a local markers for locating sea level on the waterfront throughout the day, elevation beneath the sea tells us of past sea level through the continental shelf as well as the size of the ocean basin and how much water it can hold, and the variation in height of the surface of the sea around the world lets us look at gravitational pull.
The shoreline is the narrow area between land and water. Its location changes throughout the day with tides, currents, and winds but its average position defines local (or relative) sea-level, called mean sea-level. Both global and local factors can change the height of relative sea-level such as the expansion of the oceans as they warm and any local post-glacial land movements or change in sediment input from local streams and waterways.
Seafloor features such as underwater volcanoes and mountains, deep sea trenches, and shallow continental shelves are constantly but slowly changing. Any change in the volumetric (holding) capacity of the ocean basins produces a corresponding change in sea-level. Additionally we can look at the underwater shelves that rim our continents to see past sea level.
Uneven mass distributions on the seafloor and inside Earth produce local and regional variations in gravitational attraction. The greater the mass in an area, the higher the elevation field and the more water that will be attracted there. Thus the position of the ocean surface is strongly influenced by those gravitational differences, as revealed by broad undulations in the height of the average sea surface when measured from Earth's center.