Relating to the tide — the way the sea rises and falls twice a day. A tidal river is one the sea pushes up into, so its water level changes; a tidal island can be walked to at low water and is cut off at high water; tidal power is electricity made from that movement. Because the tide is the clearest everyday example of something huge that comes and goes on its own schedule, people also use "tidal" for anything that arrives in a great unstoppable wave — "a tidal surge of orders". Note one important trap: "tidal wave" is the old, popular name for what scientists now call a tsunami, and a tsunami has nothing to do with tides at all; it comes from an earthquake. Scientists dislike the term for that reason, but ordinary people still say it, and "a tidal wave of criticism" is completely normal English for a flood of complaints.