The OED says: "The process by which the results of an insect's activity act as a stimulus to further activity."
Wikipedia is more eloquent: "...a method of communication in decentralised systems in which the individual parts of the system communicate with one another by modifying their local environment."
This meme will probably be pretty hot for a while. A quick google search finds 1200 hits for 'stigmergy' (and nearly 400 for 'stigmergy web'; a googe related: search has 24 today).