According to the Anti-Oppression Network, Allyship is "an active, consistent, and arduous practice of unlearning and re-evaluating, in which a person in a position of privilege and power seeks to operate in solidarity with a marginalized group" (2015). An Ally is someone who acts out of responsibility rather than guilt and who is interested in challenging larger oppressive power structures.
From the Anti-Oppression Network's Allyship webpage.