From AMS Glossary
(Sometimes called heat transfer coefficient.) A nondimensional number arising in the problem of heat transfer in fluids.
It may be written
where Q is the quantity of heat transferred in unit time from an immersed body across an area S, ΔT a characteristic temperature difference, L a characteristic length, and k the thermal conductivity. With suitable choices of characteristic quantities, it may be interpreted as a ratio of heat actually transferred to heat that would be transferred under the circumstances by pure conduction.