While incest is legally defined in many places as sexual activity between people who are closely and biologically related, in casual speech it often refers to the sexual abuse of a child.

Survivors of Incest Anonymous offers one very inclusive definition, defining incest as

a sexual encounter by a family member, or by an extended family member that damaged the child. By 'extended family' member we mean a person that you and/or your family has known over a period of time. This may be any family member, a family friend, clergy, another child, or anyone that betrayed the child's innocence and trust. We believe we were affected by the abuse whether it occurred once or many times since the damage was incurred immediately. By 'abuse' we mean any sexual behavior or contact with the child. Sexual contacts may include a variety of verbal and/or physical behaviors; penetration is not necessary for the experience to count as incest.

The purpose of such a broad definition is not to suggest that the laws regarding incest of any kind should be changed or that all acts of incest are the same. The broader definitions of incest are used in self-help, therapy, and other situations where the emphasis needs to be on acknowledging the effect of the abuse on a child. In that context, it becomes important to think about the connections between sexual abuse from a teacher and sexual abuse from a parent, as well as the differences.

This broader definition is also useful because it allows room for people who don't know who their abuser was, only that they were abused, or for people who need to heal from the betrayal of trust and the physical and emotional results of being abused by anyone in a position of power. It creates an inclusive atmosphere, where otherwise there might be no support for people who had been sexually abused by someone who happened not to be in their immediate family. Survivors of Incest Anonymous also has an excellent page describing the effects of abuse at http://siawso.org/effects.html.