Sentence:  The waves are separated by intervals of 15 minutes to an hour or more ( because of their great length ) , and this has often lulled people into thinking after the first great <WSD>wave</WSD> has crashed that it is all over . 
Meaning of the word marked with <WSD> tags:  one of a series of ridges that moves across the surface of a liquid (especially across a large body of water)