According to the female incelosphere, an omega female is the more technical term for a woman who is at the bottom of the totem pole in terms of SMV, LMS, or the decile scale. In other words, she's a femcel.
In terms of behaviour, she is much more submissive than a alpha female. For example, she is unlikely to be too direct. She is also likely to have confidence issues such as low self-esteem, social anxiety etc.