This paper proposes a definition of systems thinking for use in a wide variety of disciplines, with particular emphasis on the development and assessment of systems thinking educational efforts. The definition was derived from a review of the systems thinking literature combined with the application of systems thinking to itself. Many different definitions of systems thinking can be found throughout the systems community, but key components of a singular definition can be distilled from the literature. The author considered these components both individually and holistically, then proposed a new definition of systems thinking that integrates these components as a system. The definition was tested for fidelity against a System Test and against three widely accepted system archetypes. Systems thinking is widely believed to be critical in handling the complexity facing the world in the coming decades; however, it still resides in the educational margins. In order for this important skill to receive mainstream educational attention, a complete definition is required. Such a definition has not yet been established. This research is an attempt to rectify this deficiency by providing such a definition. The paper discusses various existing definitions of systems thinking, including those by Barry Richmond, Peter Senge, Linda Sweeney and John Sterman, Megan Hopper and Krystyna Stave, Birgit Kopainsky, Stephen M. Alessi, and Pål I. Davidsen, as well as Jay Forrester. These definitions often fail to capture the systemic essence of systems thinking, as they focus on elements or skills rather than defining systems thinking as a system. The author proposes a new definition of systems thinking as a system, which includes a clear goal, elements of systems thinking, and descriptions of interconnections between these elements. The definition is tested against the System Test and is found to pass all three criteria. The proposed definition includes a clear goal, detailed elements, and descriptions of interconnections between these elements. It synthesizes the most common and critical systems thinking competencies discussed in the literature. The definition can be used for systems thinking educational efforts, systems science, and a myriad other disciplines which require the use of critical systems understanding and intuition.This paper proposes a definition of systems thinking for use in a wide variety of disciplines, with particular emphasis on the development and assessment of systems thinking educational efforts. The definition was derived from a review of the systems thinking literature combined with the application of systems thinking to itself. Many different definitions of systems thinking can be found throughout the systems community, but key components of a singular definition can be distilled from the literature. The author considered these components both individually and holistically, then proposed a new definition of systems thinking that integrates these components as a system. The definition was tested for fidelity against a System Test and against three widely accepted system archetypes. Systems thinking is widely believed to be critical in handling the complexity facing the world in the coming decades; however, it still resides in the educational margins. In order for this important skill to receive mainstream educational attention, a complete definition is required. Such a definition has not yet been established. This research is an attempt to rectify this deficiency by providing such a definition. The paper discusses various existing definitions of systems thinking, including those by Barry Richmond, Peter Senge, Linda Sweeney and John Sterman, Megan Hopper and Krystyna Stave, Birgit Kopainsky, Stephen M. Alessi, and Pål I. Davidsen, as well as Jay Forrester. These definitions often fail to capture the systemic essence of systems thinking, as they focus on elements or skills rather than defining systems thinking as a system. The author proposes a new definition of systems thinking as a system, which includes a clear goal, elements of systems thinking, and descriptions of interconnections between these elements. The definition is tested against the System Test and is found to pass all three criteria. The proposed definition includes a clear goal, detailed elements, and descriptions of interconnections between these elements. It synthesizes the most common and critical systems thinking competencies discussed in the literature. The definition can be used for systems thinking educational efforts, systems science, and a myriad other disciplines which require the use of critical systems understanding and intuition.