1. A lesbian reference when specifically wanting to note the breasts of another lesbian.
2. A Lesbian that is mainly interested in breasts.
3. A straight woman that spends an above average amount of time admiring other womens breasts.
All that straight girl ever talks about is other girls boobs, she must be a breastbian.
Yeah she is a lesbian but all she really wants is to play with boobs, she is a real breastbian.