1. In mathematics, a system of differential equations and supplementary conditions such that the values of all the unknowns (dependent variables) of the system are mathematically determined for all values of the independent variables (usually space and time) to which the system applies. 2. In thermodynamics, a system of fixed mass. By some definitions such a system may be confined to an impermeable container. An idealized air parcel, not diffusing into or mixing with its environment and undergoing a saturation- adiabatic process, is closed, as contrasted to one undergoing pseudoadiabatic expansion with precipitation removed. Compare open system, isolated system. 3. In synoptic meteorology, loosely used for a closed low or closed high.