1. n. [Geophysics]

The ratio of stress to strain, abbreviated as K. The bulk modulus is an elastic constant equal to the change in applied pressure (∂P) divided by the ratio of the change in volume to the original volume of a body (∂V/V).

K = −V (∂P / ∂V),

where
K = bulk modulus
V = volume
∂P = partial derivative of pressure
∂V = partial derivative of volume.

Alternate Form: modulus of compression

See related terms: elastic constants,  Lamé constant