The most famous definition of situation awareness (SA) is the three-level model proposed by Endsley (1995):
the perception of environmental elements with respect to time or space [(Level 1 SA)], the comprehension of their meaning [(Level 2 SA)], and the projection of their status in the near future [(Level 3 SA)].
However, the definition of Smith and Hancock (1995) is based on Neisser's perceptual-cycle model (Neisser, 1976) and states that SA is:
adaptive, externally-directed consciousness that has as its products knowledge about a dynamic task environment and directed action within that environment.
How do these models differ from each other, and do these differences have practical implications?
Ps. Many more definitions of SA exist. These are all summarized in this document.
Endsley, M. R. (1995). Toward a theory of situation awareness in dynamic systems. Human Factors: The Journal of the Human Factors and Ergonomics Society, 37(1), 32-64.
Neisser, U. (1976). Cognition and reality: Principles and implications of cognitive psychology. WH Freeman/Times Books/Henry Holt & Co. ISO 690
Smith, K., & Hancock, P. A. (1995). Situation awareness is adaptive, externally directed consciousness. Human Factors: The Journal of the Human Factors and Ergonomics Society, 37(1), 137-148.