1. An image that appears distorted unless it is viewed from a special angle or with a special instrument; or the production of a distorted image by an optical system.
2. An evolutionary increase in complexity of form and function.
3. A gradually ascending progression or change of form from one type to another in the evolution of a group of animals or plants.
4. In certain arthropods, metamorphosis in which body parts or segments are added to those already present.