Anchoring is the term used to describe the effect whereby an initial exposure to information or a number serves as a reference point and influences subsequent judgments. This influence usually occurs without our awareness.
Example: A supermarket in the US had a sales promotion for Campbell' soup at about 10% off the regular price. On some days, the store posted a sign on the shelf that said "Limit of 12 per person". On other days the sign said "No limit per person". The results: Shoppers purchased on average 7 cans when the sign "Limit of 12 per person" was posted and about 3.5 cans when the sign said "No limit per person". The number "12" served as an anchor and influenced people's decision on how many cans to purchase.
There is an entire chapter devoted to Anchoring in the book "Thinking, Fast and Slow" by Daniel Kahneman. The book is rather long but if you want to learn just about anchoring, Chapter 11 focuses on Anchors.