In biology, an organ (Latin: organum, "instrument, tool", from Greek όργανον - organon, "organ, instrument, tool") is a group of tissues that perform a specific function or group of functions. Usually there is a main tissue an sporadic tissues. The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are the nerves, blood, connective etc.