A system of assumptions, concepts and relationships between them allowing to describe (model) in an approximate way a specific aspect of reality.

Standard glossary of terms used in Requirements Engineering [-]

In its most general form: relation between dependent (target) variables and independent (control) variables of a quantity. If the variables are scaled metrically, one talks about “function”; while if they exist in semantic format (non-metric), one talks about “relation”.

VDI/VDE 3682 [-]