Donald Norman coined the term -- "an artificial device designed to maintain, display, or operate upon information in order to serve a representational function" (see below for reference).
The term is used in context of research on distributed cognition. Distributed cognition is the Cognitive Science version of sociology -- examining the way people think and interact in groups.
Cognitive artifacts help offload some of the work of thinking onto an external tool, thereby freeing the human to examine other aspects of the problem. A simple cognitive artifact may serve as external memory. Written language for the purposes of factual or historical preservation is a prime example. A more complicated cognitive artifact, such as a calculator, may assist in computational functions.
Donald A. Norman (1991). Cognitive artifacts. In John M. Carroll, editor, Designing Interaction. Psychology at the Human-Computer Interface, pages 17-38. Cambridge University Press, Cambridge.