Advection is the transport of something in a fluid as the fluid moves, e.g., through expansion, convection, or gravity. The "something" transported can be a substance, or a condition, e.g., pollutant particles, water vapor, gas makeup, heat energy, or electric charge. The term can be used for liquids, such as in rivers or oceans, or gases, such as in atmospheres or gas clouds.
An advection timescale is proportional to an advection length scale (scale of the distance the material is being transported) divided by an advection velocity scale (scale of the fluid velocity). It is useful for comparison with competing timescales in determining whether advection is the principle process resulting from some material's insertion/presence in a fluid.