Used to describe behaviors or situations that differ from the socially accepted norm. Wearing a costume to a formal event would be considered an abnormality.
Pro Tip 2/3
Not Negative Always
Can describe unique traits that are not necessarily bad, just uncommon. Her ability to remember dates so clearly was an interesting abnormality.
Pro Tip 3/3
Cultural Sensitivity
What's abnormal in one culture might be normal in another; use with awareness of cultural differences. He found the local customs to be abnormalities, but later understood their cultural significance.