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. A white subdwarf orbited by four planets, with only the inner planet inhabited, with a population of 5 billion.
According to the Star Trek: Star Charts (pg. 66), this system was located in or near the Romulan War Theater of Operations (2156-2160). Romii was a K-class star with a magnitude of +5, which was the same brightness as Sol.