The definitions below refer to Adaptation to Climate Change.
The RESIN project  gives the following definition:
The positive effects that a policy or measure aimed at one objective might have on other objectives, irrespective of the net effect on overall social welfare. Co-benefits are often subject to uncertainty and depend on local circumstances and implementation practices, among other factors. Co-benefits are also referred to as ancillary benefits. 
Other International Definitions
- ↑ http://www.resin-cities.eu/
- ↑ Allaby, M., 2004. A Dictionary of Ecology. Oxford University Press, Oxford.