The misleading older term for tsunami is “tidal wave,” because tides do not cause tsunamis.
Tides are the regular rise and fall of ocean levels driven mainly by the gravitational pull of the Moon and Sun. Tsunamis are instead produced by the sudden displacement of a large volume of water, usually by an undersea earthquake, but sometimes by a landslide, volcanic eruption, explosion, glacier collapse, or meteorite impact.
The term “tidal wave” became popular because a tsunami can look like an unusually rapid or powerful tide as it moves inland. Scientists discourage it because it suggests the wrong mechanism. “Seismic sea wave” was once preferred in English, but that term is also incomplete because not every tsunami is caused by an earthquake.
The Japanese word tsunami means “harbour wave.” In deep water, a tsunami may be only around a meter high yet travel more than 800 km/h; near shore, it slows and grows dramatically. The first wave is not necessarily the largest.