Confirmation Bias refers to a type of selective thinking whereby one tends to notice and to look for what confirms one's beliefs, and to ignore, not look for, or undervalue the relevance of what contradicts one's beliefs.
In other words, it's the tendency to look for evidence that is consistent with one's preconception's and to avoid, downplay, or not notice information that is inconsistent with one's prior ideas. In psychology, confirmation bias is a phenomenon whereby, in a variety of settings, decision makers have been shown to notice more, assign more weight to, and actively seek out evidence that confirms their claims, and tend to ignore and not seek out evidence which might discount their claims. As such, it can be thought of as a form of selection bias in collecting evidence.