the quality of being difficult to do; "he assigned a series of problems of increasing hardness"; "the ruggedness of his exams caused half the class to fail"
the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale
a wave resulting from the periodic flow of the tides that is caused by the gravitational attraction of the moon and sun
an unusual (and often destructive) rise of water along the seashore caused by a storm or a combination of wind and high tide
an overwhelming manifestation of some emotion or phenomenon; "a tidal wave of nausea"; "the flood of letters hit him with the force of a tidal wave"; "a tidal wave of crime"