What term describes a common belief that is oversimplified and generalized about a group?

The term that describes a common belief that is oversimplified and generalized about a group is "stereotype." Stereotypes are widely held but fixed and oversimplified images or ideas of a particular type of person or thing. They often result from cognitive shortcuts and can lead to misconceptions or inaccurate representations of groups. For instance, if a stereotype suggests that all members of a particular group share the same traits or behaviors, it ignores individual differences and nuances, simplifying the rich diversity of experiences and identities within that group.

In contrast, the other terms do not capture this idea. Bias refers to a tendency to favor one group over another, often leading to unfair judgments or decisions, but does not inherently imply oversimplification or generalization. A fallacy is an error in reasoning that undermines the logical validity of an argument, and it does not specifically pertain to generalizations about groups. A fact is a statement that can be proven true or false and does not convey the generalized nature that stereotypes do. Therefore, "stereotype" is the most accurate descriptor in the context of the question.