A definition of cognition from the textbook that coined the term "cognitive psychology":

[T]he term "cognition" refers to all the processes by which the sensory input is transformed, reduced, elaborated, stored, recovered, and used. It is concerned with these processes even when they operate in the absence of relevant stimulation, as in images and hallucinations. Such terms as sensation, perception, imagery, retention, recall, problem solving, and thinking, among many others, refer to hypothetical stages or aspects of cognition.

Ulric Neisser (1967). Cognitive Psychology. New York: Appleton-Century-Crofts. (p. 4)