Biot Number is a dimensionless number which is the ratio of internal thermal resistance of solid to fluid thermal resistance. This is used for heat transfer between fluids and solids.
\(Bi\) = biot number
\(h\) = film coefficient or heat transfer coefficient or convective heat transfer coefficient
\(l_c\) = characteristic length
\(k\) = thermal conductivity of the body
\(c_m\) = mass transfer coefficient
\(c_d\) = diffusion coefficient