Romii was the primary of the Romii system. This Romulan system was in Sector Z-6, and was located near the Romulan Neutral Zone. This sector was a region of space in the Alpha Quadrant. (TOS: "Balance of Terror", display graphic)
According to Star Trek Maps (pg.29), Romii was an early Romulan colony. It was a white subdwarf orbited by four planets, with only the inner planet inhabited, with a population of five billion.