A piece of information that can be used to accomplish tasks, perhaps along with other knowledge components. Knowledge component is a generalization of everyday terms like concept, principle, fact, or skill, and cognitive science terms like schema, production rule, misconception, or facet. A knowledge component (KC) relates features to a response where both the features and response(s) can be either external, in the world, like cues in a stimulus and a motor response or internal, in the mind, like inferred features and a new goal.
KCs are "correct" when all of the features are relevant to making the response and none of them are irrelevant. In geometry, for example, the knowledge component "if angles look equal, then conclude they are equal" is incorrect because it includes an irrelevant feature "angles look equal" and is missing a relevant feature like "the angles are at the base of isosceles triangle".
For further discussion and examples see:
1) VanLehn, 2006, "The Behavior of Tutoring Systems"
2) Koedinger's PSLC Lunch Talk from August, 2006.
3) Norma Chang's CMU Psychology PhD Thesis (2006) on surface vs. structural probem variations and resultant acquisition of relevant vs. irrelevant features ("spurious correlations" with surface features).