A cewar ƙamus, hydroxyl ion wani nau'in ƙwayar cuta ne ko ion da aka yi mara kyau wanda ya ƙunshi hydrogen atom ɗaya da oxygen atom ɗaya, wanda galibi ana wakilta shi azaman OH-. Yana samuwa ne lokacin da kwayoyin ruwa ya rasa hydrogen ion, wanda ya haifar da ion hydroxide (OH-) da kuma ion hydrogen (H). Hydroxyl ions suna da mahimmanci a yawancin halayen sinadarai kuma suna taka muhimmiyar rawa a cikin acidity ko asali (pH) na bayani.