Three definitions:
Gestalt principles, or gestalt laws, are rules of the organization of perceptual scenes.
http://www.scholarpedia.org/article/Gestalt_principles
This idea of seeing the whole before the parts and even more the whole becoming more than the sum of its parts is Gestalt.
http://www.vanseodesign.com/web-design/gestalt-principles-of-perception/
Gestalt principles of form perception: Gestalt psychology attempts to understand psychological phenomena by viewing them as organized and structured wholes rather than the sum of their constituent parts. Thus, Gestalt psychology dissociates itself from the more ‘elementistic’/reductionistic/decompositional approaches to psychology like structuralism (with its tendency to analyze mental processes into elementary sensations) and it accentuates concepts like emergent properties, holism, and context.
http://www.interaction-design.org/encyclopedia/gestalt_principles_of_form_perception.html
My attempt at illustrating Gestalt:
