An animal subject or group in a scientific experiment where the factor being tested is not applied so that it may serve as a standard for comparison against another group where the factor is applied.
A control group is useful to make conclusions more accurate or precise, provided that both the control and the other experimental group(s) are exposed to same conditions apart from the factor being tested. For instance, a control animal is one in which the drug is not administered to.
It is not unusual that this involves animals due to practical reasons, e.g. limited number of human subjects and ethical concerns.