A process of exposing insects, fungal spores or other organisms to the fumes of a chemical at a lethal strength in an enclosed space for a given period of time: Fumigant is a chemical, which at a required temperature and pressure can exist in the gaseous state in sufficient concentration to be lethal to a given pest organism.  
Example: Use of acetic acid is mandatory as post harvest fumigant to destroy fungal spores on peaches, nectarines, apricots, and cherries; methyl bromide for fumigating cut flowers and many other commodities.