Subject dictionaries are useful tools for looking up definitions of unfamiliar terms, and several of these, which focus specifically on environmental and natural resources, are linked below:
The New Palgrave Dictionary of Economics
This is the Living Edition (dynamic online version) of The New Palgrave Dictionary of Economics. Written by leading figures in the field including many Nobel prize winners, this is the definitive scholarly reference work for a new generation of economists. Regularly updated!
Subject encyclopedias are useful for obtaining overviews of topics in a particular subject area or field of study. The following titles are recommended sources:
Encyclopedia of Energy, Natural Resource, and Environmental Economics by 
Environmental and Natural Resource Economics: An Encyclopedia by 