From AMS Glossary
Approximations made to the Reynolds-averaged equations of turbulence to allow solutions for flow and turbulence variables.
The Reynolds-averaged equations contain statistical correlations such as the variance or covariance between dependent variables such as velocity or temperature. The equations that forecast lower-order correlations often contain unknowns of higher statistical order, a difficulty known as the closure problem. When the higher-order terms are approximated as empirical functions of lower-order terms and of known independent variables, the resulting approximate equations can then be solved. These approximations, known as closure assumptions, must satisfy parameterization rules.
Stull, R. B. 1988. An Introduction to Boundary Layer Meteorology. 666 pp.