na-adjective
irregular; atypical; anomalous
Describes something that deviates from the normal rules, patterns, or standards. Often used for procedures, methods, or situations that are not standard.
変則的な日程で会議が行われた。
The meeting was held on an irregular schedule.
彼のやり方は少し変
His method is a bit unorthodox, but he gets results.
異常 implies something is abnormal or unusual, often with a negative or alarming connotation, whereas 変則的 is more neutral and simply means non-standard.
Compound of 変則 (irregular rule) + 的 (-like, adjectival suffix). The base 変則 is itself composed of 変 (change, strange) and 則 (rule, norm).