please define "abuse"

Would someone please define "abuse" for me . Recently I was asked to give up my vacation for a coworker ,I said no I have a dying family member to help take care of and I am in nursing school and I would like to have my vacation days . My coworker is now saying my not giving up my vacation days, so she can have them ,is a form of abuse . She never said why she needed my vacation days , the last three years I did this she went to Atlantic city . I am told if my co-worker keeps on calling this abuse I might have to go to counciling for my abusive behavior