In science, a theory is a well-substantiated explanation of some aspect of the natural world that is acquired through the scientific method. Theories are based on evidence and observations and are constantly tested and revised as new information becomes available.

General sense:

In a more general sense, a theory is a set of principles or assumptions that explains or predicts a phenomenon or group of phenomena. It is a provisional explanation that is based on evidence but is not considered to be absolute or final.