Minamoto was asurname given by theEmperors of Japan for members of the imperial family who were excluded from the line to become next Emperor and lowered into the ranks of the common lords since 814. The Minamoto was the most powerful clan of all four great clans that dominated Japanese politics during the Heian,Kamakura,Muromachi andEdo periods in Japanese history. The other three were theFujiwara, theTaira, and theTachibana.
The Minamoto clan is also called theGenji (源氏, "Minamoto clan"), or sometimes theGenke (源家, "House of Minamoto"), using theOn'yomi readings ofgen (源) for "Minamoto", whileshi orji (氏) means "clan", andke (家) is used to read "extended family".
The Taira, their rival were actually Emperor descendant too which make them Cousin to the Minamoto.