A new empirical parameter bridging the wave steepness in deep water and Ursell's parameter in shallow water is proposed to describe the phenomena of wave nonlin
earity. Relative crest elevations, skewness, and root-mean-square surface elevation relative to wave height are all expressed as unique functions of the new parameter. Theoretical profiles of the third order Stokes waves and the second order cronidal waves, laboratory data of regular waves, and field data of sea waves in coastal waters support the relation between the nonlinear wave properties and the new parameter. Deformation of wave spectrum due to secondary interactions is also governed by the new parameter. The mean wave period derived from spectrum, the spectral peakedness parameter, and the slope of spectrum in high frequency range all decrease as the value of nonlinearity parameter increases.