For questions about living things. Does not exclude aliens, but additional information is usually necessary (consider using "xenobiology" instead).
Biology is the study of life. Questions using this tag are about organisms and their morphology, physiology, anatomy, behavior, origin, distribution, and related characteristics. This includes "what-if" scenarios for the existence of certain (often unorthodox) life forms - such questions may wish to consider using the reality-check tag as well.
You can use this tag on questions about alien biology, but keep in mind that you will need to explain your aliens in the questions. More information may be required, so keep an eye on the comments asking for details. The xenobiology tag may be more appropriate.