The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species of life, such as seastars, sea urchins, and many species of coral with regional differences in biodiversity. Sometimes it is referred to as the littoral zone or seashore, although those can be defined as a wider region.

The well-known area also includes steep rocky cliffs, sandy beaches, bogs or wetlands (e.g., vast mudflats). The area can be a narrow strip, as in Pacific islands that have only a narrow tidal range, or can include many meters of shoreline where shallow beach slopes interact with high tidal excursion. The peritidal zone is similar but somewhat wider, extending from above the highest tide level to below the lowest. Organisms in the intertidal zone are adapted to an environment of harsh extremes, living in water pressure with the potential of reaching 5,580 pounds per square inch. The intertidal zone is also home to several species from different phyla (Porifera, Annelida, Coelenterata, Mollusca, Arthropoda, etc.).

Water is available regularly with the tides that can vary from brackish waters, fresh with rain, to highly saline and dry salt, with drying between tidal inundations. Wave splash can dislodge residents from the littoral zone. With the intertidal zone's high exposure to sunlight, the temperature can range from very hot with full sunshine to near freezing in colder climates. Some microclimates in the littoral zone are moderated by local features and larger plants such as mangroves. Adaptation in the littoral zone allows the use of nutrients supplied in high volume on a regular basis from the sea, which is actively moved to the zone by tides. Edges of habitats, in this case land and sea, are themselves often significant ecologies, and the littoral zone is a prime example.

A typical rocky shore can be divided into a spray zone or splash zone (also known as the supratidal zone), which is above the spring high-tide line and is covered by water only during storms, and an intertidal zone, which lies between the high and low tidal extremes. Along most shores, the intertidal zone can be clearly separated into the following subzones: high tide zone, middle tide zone, and low tide zone. The intertidal zone is one of a number of marine biomes or habitats, including estuary, neritic, surface, and deep zones.