Which term describes behavior that is considerably different from an accepted norm or group?

The term "aberrant" refers specifically to behavior that deviates significantly from what is considered normal or accepted within a group. In various contexts, especially in psychology and sociology, aberrant behavior indicates actions or reactions that are not only unexpected but also inconsistent with societal norms or expectations. This term is often used to highlight significant variances that could lead to a deeper understanding of an individual's psychological or behavioral issues.

The other terms do have distinct meanings. "Abnormal" generally refers to anything that is not typical or expected, often used in clinical settings to describe psychological or physical conditions. However, it doesn't necessarily carry the same connotation of significant deviation from social norms that "aberrant" does. "Adventitious" relates to something occurring by chance or accident, which doesn't apply to the context of behavioral norms. "Antithetical" suggests being directly opposed or contrasting to something, but it lacks the direct implication of deviation from societal norms that "aberrant" provides. Thus, "aberrant" is the most fitting term for describing behavior that stands out markedly from what is normally accepted.