Pollution is generally considered to be the introduction, directly or indirectly, by human activities (usually) of substances or energy into the environment resulting in deleterious effects of such a nature as to endanger human health, harm living resources or ecosystems, and impair or interfere with amenities and other legitimate uses of the environment.
The definition of pollution I prefer to use for the purposes of assessing environmental impacts and sustainability follows:
"The release of any material, surplus energy or other waste stream into the environment in sufficient quantities to exceed thresholds, and adversely affect environmental quality in the short, medium or long-term".
(Further details on the corresponding context of this and many of the other terms used in this definition can also be found here).