geoid   the equipotential surface of the Earth's gravity field that best fits, in a least squares sense, global mean sea level (National Oceanic and Atmospheric Administration, 2001)

mean lower low water   the average of the lower low water height of each tidal day observed during the national tidal datum epoch (currently, 1983–2001)

mean low water   the average of all low water heights observed during the national tidal datum epoch (currently, 1983–2001)