1. Not suited to circumstances or needs; unsuitable: improper shoes for a hike; improper medical treatment.

2. Not in keeping with conventional mores; indecorous: improper behavior; an improper suggestion. See Synonyms at unseemly.

3. Not consistent with established truth, fact, or rule; incorrect: improper grammar; an improper conclusion.

4. Irregular or abnormal: complications due to improper functioning of the immune system.