Dictionary Definition: 

“The exertion of force overcoming resistance or producing molecular change.”

Selected from OxfordDictionaries.com

Functional Definition:

You may remember work from your high school or college physics class. Work is performed by the energy exerted on a system. In the case of chemical reactions energy usually comes in the form of heat transferring between systems. Work pushes reactions forward or determines how much energy is needed to cause a reaction or is produced by a reaction.