<p><strong>Noun</strong></p><ul><li>a female offspring having both parents in common with another offspring; female sibling. Also called half sister.</li><li>a female offspring having only one parent in common with another offspring; stepsister.</li><li>a female friend or protector regarded as a sister.</li><li>a thing regarded as feminine and associated as if by kinship with something else: The ships are sisters.</li><li>a female fellow member, as of a church.</li><li>a female member of a religious community that observes the simple vows of poverty, chastity, and obedience.</li><li>British: a nurse in charge of a hospital ward; head nurse.</li><li>Informal: a term used to refer to or address a fellow Black woman; soul sister.</li><li>a woman who supports, promotes, or participates in feminism.</li><li>Informal: a form of address used to a woman or girl, especially jocularly or contemptuously: Listen, sister, you've had enough.</li></ul><p><strong>Adjective</strong></p><ul><li>being or considered a sister; related by or as if by sisterhood: sister ships.</li><li>having a close relationship with another because of shared interests, problems, or the like: We correspond with school children in our sister city.</li><li>Biochemistry: being one of an identical pair.</li></ul>