Anchoring and adjustment heuristic is a mental shortcut that involves using a number or value as a starting point, and then adjusting one's answer away from this anchor; people often do not adjust their answer sufficiently.
People who have to make judgements under uncertainty use this heuristic by starting with a certain reference point (anchor) and then adjust it insufficiently to reach a final conclusion. Example: If you have to judge another person ´s knowledge, the anchor for your final (adjusted) judgement may be your own level of knowledge. Depending on your own level of knowledge you might therefore underestimate or overestimate the knwledge of this person
Other /More definition:
Anchoring and adjustment heuristic refers to a tendency to make judgments by beginning with an initial estimate (an anchor) and then adjusting this estimate to reach a final decision.