A wind tunnel is an enclosed passage through which air is driven by a fan or any appropriate drive system.
Credits - NASA
The heart of the wind tunnel is the test section where a scale model is supported in a carefully controlled airstream that produces a flow of air about the model that duplicates the full-scale aircraft. Appropriate balances and test instrumentation measure the aerodynamic characteristics of the model and the field around it (its flow field). Although the form of a wind tunnel can vary, all wind tunnels have a drive system, a test section, use a model that is supported in an airstream and whose characteristics are measured by test instrumentation. The wind tunnel allows the aerodynamic forces of lift, drag, and side force in reference to the tunnel axis (the axial centerline of the test section) to be measured.