1. Bel, a dimensionless unit for expressing power or intensity ratios; log10 of the power ratio. The more common unit is the decibel (q.v.); see Figure D-3. Named for Alexander Graham Bell (1847–1922), inventor. 2. Symbol for induced magnetic field (q.v.). 3. The gradient in AVO studies, where the amplitude is often approximated by the equation A+Bsin2θ, where θ is the angle of incidence.