yottameter (Ym) A metric unit of distance equal to 1024 meters or 1021 kilometers. One yottameter equals 32.408 megaparsecs (Mpc) or 105.7 million light years. It is said that we probably do not need a longer distance unit term than the 'yottameter', because the radius of the observable universe is not more than about 200 yottameters. ...