I'll start...

first let's define the suffix "ism" :

A suffix occurring at the end of a word, base, or phrase, forming abstract nouns that denote a doctrine, system, practice, state, condition, or philosophy (Merriam-Webster, Oxford English Dictionary,

next environmental:

environmental

/ĕn-vī″rən-mĕn′tl, -vī″ərn-/

adjective

1.Relating to or associated with the environment.

2.Relating to or concerned with the impact of human activities on the natural environment or the environmental movement.

3. Relating to potentially harmful factors originating in the environment.

environmental illness

from The American Heritage® Dictionary of

the English Language, 5th Edition

Using the above, for the purpose of this conversation I'll define Environmentalism as:

"A doctrine or philosophy relating to or concerned with the impact of human activities on the natural environment."