Sentence:  <WSD>Betas</WSD> are usually second in command to the reigning alpha, and omegas belong to the lowest caste of the social hierarchy. 
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.