Kinktionary
More like guidelines and a lot less like definitions.
Beta
Beta is often used as a slang insult for or describing a man who is seen as passive, subservient, weak, and effeminate. It generally tries to define a structure together with Alpha and Sigma to describe personality type where the, mostly male, person mostly can be described as friendly, reserved, loyal, collaborative and generally a "nice guy" type.
Criticism
This word may be considered offensive by some people as it is derived from a fairly toxic masculine cultural background. There has been documentation that disproves the myth of the Alpha Male (second source). The term is also sometimes associated with manosphere groups, in particular incel groups but some people may choose to use this term as a means of reclaiming it for themselves.