1. The set of elements to which a mathematical or logical variable is limited; the set on which a function is defined; the set of values that an independent variable may take. 2. The class of terms that have a given relation to something is called the ‘‘domain of that relation.’’ Thus, when we speak of a seismic trace in the time domain, we mean that time is the independent variable; in the frequency domain, frequency is the independent variable; in the f,k domain or f,k space, frequency and wavenumber are the independent variables; etc. 3. A region of magnetic polarization in a single direction (magnetic moments parallel) that behaves as a unit during change in magnetization. The dimensions of magnetic domains are about 1μm. 4. The areal extent of a given lithology or environment. 5. The region occupied by a cluster set in cluster analysis.