A dysfunctional relationship between two people in which one person is addicted to trying to "fix" the other person.
A common behavior in codependents is enabling, in which the codependent overlooks problem behavior in the other person or makes excuses for him/her in order to continue feeling needed.
The word codependence was originally coined to describe the behavior of people living with an alcohol or substance abuser, however, the meaning has expanded to include any person who has learned maladaptive patterns of relating to others. This may be due to growing up in a family situation where emotions were repressed and other people's needs were put before one's own.

