What is Tide?

Legal Definition
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth.

The times and amplitude of tides at any given locale are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean, by the amphidromic systems of the oceans, and the shape of the coastline and near-shore bathymetry (see Timing). Some shorelines experience a semi-diurnal tide—two nearly equal high and low tides each day. Other locations experience a diurnal tide—only one high and low tide each day. A "mixed tide"—two uneven tides a day, or one high and one low—is also possible.

Tides vary on timescales ranging from hours to years due to a number of factors. To make accurate records, tide gauges at fixed stations measure water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference (or datum) level usually called mean sea level.

While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near coasts.

Tidal phenomena are not limited to the oceans, but can occur in other systems whenever a gravitational field that varies in time and space is present. For example, the solid part of the Earth is affected by tides, though this is not as easily seen as the water tidal movements.
-- Wikipedia
Legal Definition
The ebb and flow of the sea.

2. Arms of the sea, bays, creeks, coves, or rivers, where the tide ebbs and flows, are public, and all persons may use the same for the purposes of navi-gation and for fishing, unless restrained by law. To give these rights at common law, the tide must ebb and flow: the flowing of the waters of a lake into a river, and their reflowing, being not the flux and reflux of the tides, but mere occasional and rare instances of a swell in the lake, and a setting up of the waters into the river, and the subsiding of such swells, is not to be considered an ebb and flow of the tide, so as to constitute a river technically navigable. 20 John. R. 98. See 17 John. R. 195; 2 Conn. R. 481.

3. In Pennsylvania, the common law principle, that the flux and reflux of the tide ascertain the character of the river, has been rejected. 2 Binn. R. 475. Vide Arm of the sea; Navigable river; Sea shore.
-- Bouviers Law Dictionary
Legal Definition
The ebb and flow of the sea. See Baird v. Campbell, 67 App. Div. 104, 73 N. Y. Supp. 617.
-- Black's Law Dictionary