From AMS Glossary
A kinematic measure of the tendency of the flow in an air mass to increase the horizontal gradient of a conservative property θ, defined by the equation
where ∇H is the horizontal del operator.
When θ is identified with the potential temperature, this function measures the frontogenesis in an air mass.
Pettersen, S. 1956. Weather Analysis and Forecasting. 2d ed., Vol. I, . 200–201.