The father of a father is a grandfather, a grandfather's father is a great-grandfather, a great-grandfather's father is a great-great-grandfather, but no word has been decided upon for the generations before great-great-grandfather.
父の父は祖父、祖父の父は曽祖父、曽祖父の父は高祖父だが、高祖父より前の世代の呼称は決まっていない。