The structure of a system is the arrangement of its subsystems and components in three-dimensional space at a given moment of time. This always changes over time. It may remain relatively fixed for a long period or it may change from moment to moment, depending upon the characteristics of the process in the system. This process halted at any given moment, as when motion is frozen by a high-speed photograph, reveals the three-dimensional spatial arrangement of the system's components as of that instant.

p. 22; As cited in: Egolfs Voldemars Bakuzis (1974) Foundations of Forest Ecosystems: Concepts of systems in general. p. 490 - Living systems, 1978