Sentence:  Many A/B/O stories posit societies where biological imperatives divide people based on wolf pack hierarchies into sexual dominants (alphas), sexual submissives (omegas), and everyone else (<WSD>betas</WSD>). 
Meaning of the word marked with <WSD> tags:  In omegaverse fan fiction, a person of a (usually secondary) gender/sex that mirrors the biology of a normal human, being free from the biological drives of alphas and omegas but generally capable of bonding and mating with either.