Mean low water is a term commonly used in coastal engineering and navigation, which refers to the average height of low tide over a certain period of time. There are several synonyms for mean low water, including Chart Datum, MLLW, Low Water Reference Plane, and Lowest Astronomical Tide. The term Chart Datum is used in nautical charts to denote the depth of water and is often used interchangeably with mean low water. MLLW, or Mean Lower Low Water, is a similar term that is used in a more technical context in hydrology and is defined as the average of the lowest tide levels observed over a given period of time. Both Low Water Reference Plane and Lowest Astronomical Tide are used in different contexts to define a similar reference level for water depths at specific locations.