The term “tidal wave” is used in several different ways inside and outside the oceanographic community. Technically, tidal waves are the crests of the tides which move across the surface of the Earth, and this is the usage of the term which is preferred by oceanographers and other people who study water and the movement of the ocean. However, many laypeople refer to other types of waves, such as storm surge and tsunamis, as tidal waves.
In the case of the tidal crest, tidal waves are constantly moving across the surface of the Earth as the Earth's water rises and falls, creating the tides. The size of a tidal crest can vary, depending on the relationship between the Earth and the moon and the area. The term “tidal wave” is also used to discuss tidal bore, a phenomenon which occurs in areas with large extremes between low and high tides. In these regions, when tides enter a narrow inlet, a large wave develops as the tide floods in. The tidal bore can flow up rivers and streams as it floods into land, often moving against a strong current.
For laypeople, “tidal waves” are extremely large waves caused by unusual weather conditions. Storm surge involves radical increases in the height of the ocean, along with very rough waves, caused by stormy and intense conditions. Many hurricanes are accompanied with storm surge which can overcome levies and other barriers which are designed to keep the water back, and this flood of water can cause significant damage, especially in low-lying communities.
Tsunamis are created when large amounts of water are displaced. Classically, a tsunami occurs when an earthquake shifts the water, creating a powerful wave or set of waves. Tsunamis are especially common in the Pacific Ocean, around the famous “ring of fire” which is noted for its high levels of geologic activity. Tsunamis can also be caused by volcanic eruptions when these eruptions appear close to the water.
Some people also refer to rogue or freak waves as tidal waves. These waves typically occur out in the open ocean, and the reasons for their development are a bit unclear. However these waves form, they can become extremely large. Some researchers suspect that the mysterious disappearances of ships could be explained by rogue waves which overwhelmed the craft while they were underway.
Tsunamis, rogue waves, and storm surge have nothing to do with the tides, making “tidal wave” an inappropriate term to use to refer to them. However, the widespread misusage of this term is unlikely to fade, and many oceanographers have given up on trying to educate people about which terms to use, especially since the media is especially fond of describing any unusual wave as a tidal wave, whether or not the tides are involved.