You might have bumped into this term while trying to find out when you’ll experience high tide where you live. Many coastal places actually have two high tides a day. Mean Higher High Water (MHHW) is the average of the higher of the two daily high tides. The term MHHW is more precise than “high tide,” and it helps specialists in the field communicate clearly about the tides they track. Problematic flooding can happen when water levels climb even a bit higher than normal variations in MHHW at a particular location. This is commonly called high tide flooding, and it can inundate roads, compromise stormwater systems, and damage coastal property.
While some places have one high tide and one low tide per day, most coastal locations have two high tides and two low tides a day. These highs and lows typically aren't equal. This is why, in most places, using the phrase "high tide" might be unclear. There's actually high tide and higher high tide (and low and lower low tide).
If the Earth were a perfect sphere without large continents, and if the earth-moon-sun system were in perfect alignment, every place would get two equal high and low tides every day. However, the alignment of the moon and sun relative to Earth, the presence of the continents, regional geography, and features on the seafloor, among other factors, make tidal patterns more complex.
Around the world, there are three basic tidal patterns: semidiurnal, mixed, and diurnal. When both high tides are about equal to each other, and the low tides are also roughly equal, the pattern is called a semidiurnal tide. If the two highs and lows differ substantially, the pattern is called a mixed tide. Where there's only one high and one low tide a day, it's called a diurnal tide. One location can experience different tide patterns throughout the month.