In forced convection, the fluid has a nonzero streaming motion in the farfield away from the body surface, caused perhpas by a pump or fan or other driving force independent of the presence of the body. Two major examples are duct flows and bodies immersed in a uniform stream. Also in this class are bodies moving through a still fluid, since an observer on the body would see a streaming motion in the farfield. Since fluid velocities are forced and may be large, heat transfer via forced convection will usually be significantly larger than that in natural convection.
[from White, Frank M., Heat Transfer]