What is the term for a deviation from the normal or common order or form or rule?

The term that best describes a deviation from the normal or common order, form, or rule is "anomaly." An "anomaly" specifically refers to something that deviates from what is standard, normal, or expected. It carries a connotation of something being unusual or atypical in a significant way, making it particularly suitable when referring to deviations from established patterns or rules.

While "deviation" also implies a departure from the norm, it is a broader term that does not inherently convey the sense of unusualness that "anomaly" does. "Exception" typically refers to something that does not follow a rule or pattern but does not necessarily imply that it is atypical or noteworthy. Lastly, "irregularity" indicates something that is not regular or consistent, which could suggest deviations but lacks the specific implication of being an unusual occurrence like "anomaly" does. Thus, "anomaly" is the most fitting term to describe a significant deviation from the expected norm.