Sub-disciplines of biology are defined by the research methods employed and the kind of system studied: theoretical biology uses mathematical methods to formulate quantitative models while experimental biology performs empirical experiments to test the validity of proposed theories and understand the mechanisms underlying life and how it appeared and evolved from non-living matter about 4 billion years ago through a gradual increase in the complexity of the system. See branches of biology.
Biology derives from the Ancient Greek words of βίος; romanized bíos meaning "life" and -λογία; romanized logía (-logy) meaning "branch of study" or "to speak". Those combined make the Greek word βιολογία; romanized biología meaning biology. Although this, the term βιολογία as a whole didn't exist in Ancient Greek. First to borrow it were the English and French biologie. Since the advent of the scientific era, reanalyzable as a compound using the combining forms bio + logy.