In fluid dynamics, the lift coefficient (C_L) is a dimensionless quantity that relates the lift generated by a lifting body to the fluid density around the body, the fluid velocity and an associated reference area. A lifting body is a foil or a complete foil-bearing body such as a fixed-wing aircraft. C_L is a function of the angle of the body to the flow, its Reynolds number and its Mach number. The section lift coefficient c_l refers to the dynamic lift characteristics of a two-dimensional foil section, with the reference area replaced by the foil chord.

The lift coefficient C_L is defined by

where ${\displaystyle L\,}$ is the lift force, ${\displaystyle S\,}$ is the relevant surface area and ${\displaystyle q\,}$ is the fluid dynamic pressure, in turn linked to the fluid density ${\displaystyle \rho \,}$, and to the flow speed ${\displaystyle u\,}$. The choice of the reference surface should be specified since it is arbitrary. For example, for cylindric profiles (the 3D extrusion of an airfoil in the spanwise direction), the first axis generating the surface is always in the spanwise direction. In aerodynamics and thin airfoil theory, the second axis is commonly in the chordwise direction:

resulting in a coefficient:

While in marine dynamics and for thick airfoils, the second axis is sometimes taken in the thickness direction:

resulting in a different coefficient:

The ratio between these two coefficients is the thickness ratio:

The lift coefficient can be approximated using the lifting-line theory, numerically calculated or measured in a wind tunnel test of a complete aircraft configuration.