Numerical constant that describes the elastic properties of a solid or fluid under pressure from all sides. It is the ratio of the tensile strength or compressive force per unit surface area to the change in volume per unit volume of the solid or fluid and thus is a measure of a substance's ability to resist deformation. Its units are newtons per square metre (N/m2). Matter that is difficult to compress has a large bulk modulus; for example, steel has a bulk modulus of 1.6 × 1011 N/m2, three times that of glass (i.e., glass is three times more compressible than steel).
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