1. n. 
The ratio of amplitude of the reflected wave to the incident wave, or how much energy is reflected. If the wave has normal incidence, then its reflection coefficient can be expressed as:
R = (ρ2V2 − ρ1V1) / (ρ2V2 + ρ1V1),
R = reflection coefficient, whose values range from −1 to +1
ρ1 = density of medium 1
ρ2 = density of medium 2
V1 = velocity of medium 1
V2 = velocity of medium 2.
Typical values of R are approximately −1 from water to air, meaning that nearly 100% of the energy is reflected and none is transmitted; ~0.5 from water to rock; and ~0.2 for shale to sand. At non-normal incidence, the reflection coefficient defined as a ratio of amplitudes depends on other parameters, such as the shear velocities, and is described as a function of incident angle by the Zoeppritz equations.
See: acoustic impedance, acoustic impedance section, amplitude, density, density contrast, embedded wavelet, impedance, normal incidence, reflection, reflection tomography, Ricker wavelet, Snell's law, synthetic seismogram, velocity, wave, Zoeppritz equations