1. In thermodynamics, a system so chosen that no transfer of mass takes place across its boundaries; for example, a fluid parcel undergoing a saturation-adiabatic process, as opposed to a pseudoadiabatic expansion. See open system.
2. 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.
3. = closed ecological system.
4. A system which constitutes a feedback loop so that the inputs and controls depend on the resulting output. For example, an automatic radar-controlled tracking system.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use