noun

1. a thermodynamic measure of the amount of energy unavailable for useful work in a system undergoing change
2. a measure of the degree of disorder in a substance or a system: entropy always increases and available energy diminishes in a closed system, as the universe
3. in information theory, a measure of the information content of a message evaluated as to its uncertainty
4. a process of degeneration marked variously by increasing degrees of uncertainty, disorder, fragmentation, chaos, etc.; specif., such a process regarded as the inevitable, terminal stage in the life of a social system or structure

Etymology: Ger entropie, arbitrary use (by R. J. E. Clausius, 1822-88, Ger physicist) of Gr entropē, a turning toward, as if < Ger en(ergie), energy + Gr tropē, a turning: see trope