an irregular galaxy that has an abnormal shape (neither elliptical, spiral, nor lenticular) and/or has another unusual characteristic , like jets of gas spewing from the nucleus, unusual amounts of dust, low surface brightness, etc
Any galaxy not classified as elliptical, irregular or spiral. Most peculiar galaxies appear to be interacting gravitationally with other galaxies. The Ringtail, NGC 4038 and NGC 4039, is an interacting galaxy pair.
A peculiar galaxy is a galaxy which is unusual in its size, shape, or composition. Peculiar galaxies come about as a result of interactions between galaxies, and they may contain atypical amounts of dust or gas, may have higher or lower surface brightness than a typical galaxy, or may have features such as nuclear jets. They can be highly irregular in shape due to the immense gravitational forces which act on them during encounters with other galaxies.