There are many definitions of Codependency, originally it was used to describe a person whose life was affected as a result of being involved with someone who was chemically dependent. This was according to Martin Bobgan.
But today it varies greatly, these are some different definitions:

According to Melody Beattie, " A codependent person is one who has let another person's behavior affect him or her, and who is obsessed with controlling that person's behavior."

Hemfelt, Minirth, & Meier said that " Codependency can be defined as an addiction to people, behaviors or thing. It is the fallacy of trying to control interior feelings by controlling people, things, and events on the outside. To the codepended, control, or the lack of it, is central to every aspect of life. When it comes to people, the codependent has become so ealborately enmeshed in the other person that the sense of self-personal identity is severely restricted, crowded out by that other person's identity and problems."

And Bobgan said that "Codependency is a pattern of painful dependency on compulsive behaories and on approval from others in an attempt to find safety, self-worth, and identity."

There are many causes of codependency:

1. Emotional needs that are unmet - the theory on this is that all of us have a reservoir for love or what we call love tank inside us. If our love tank has not been filled by the "significant others" in our lives our emotional needs will be unmet. We will then become codependent. This is especially true in children.
2. Lost childhood - this can be due to abusive parents or parental figures. Active abuse like incest, physical abuse or excessive anger on a parent's part is the most recognized form of abuse.
3. A compulsion to fix the dysfunctional family