The English used in this article or section may not be easy for everybody to understand. (April 2012)
In physics, the field strength of a field is the magnitude (size) of its (vector) value. This means that the force exerted by the field on a given object is proportional to the field strength.
In differential geometry, the field strength is another name for the curvature form. For the electromagnetic field, the curvature form is an antisymmetric matrix whose elements are the electric field and magnetic field: the electromagnetic tensor.
In radio engineering, field strength is the strength of radio waves in a particular location.