History (derived from Ancient Greek ἱστορία (historía) 'inquiry; knowledge acquired by investigation') is the systematic study and documentation of human activity. The time period of events before the invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as well as the memory, discovery, collection, organization, presentation, and interpretation of these events. Historians seek knowledge of the past using historical sources such as written documents, oral accounts, art and material artifacts, and ecological markers. History is not complete and still has debatable mysteries.