Robert Burney once coined the term "co-dependent" in the 1940s and defined it as someone who depends on another person to meet their emotional needs. Co-dependency is putting somebody else's needs before your own.

Thereby, being co-dependent is hardly the same thing as simply being dependent.  A co-dependent person may feel responsible for the happiness or well-being of others — especially if they have experienced abuse or neglect growing up. 

This term was widely used in the 1950s in the context of Alcoholics Anonymous, and Co-dependency behaviour is often found in alcoholic families.