Emergence is the idea that complex behavior can arise from simple, local rules. Here are a few more complete definitions, with links to their sources:

"Emergence is what happens when the whole is smarter than the sum of its parts. It's what happens when you have a system of relatively simple-minded component parts -- often there are thousands or millions of them -- and they interact in relatively simple ways. And yet somehow out of all this interaction some higher level structure or intelligence appears, usually without any master planner calling the shots. These kinds of systems tend to evolve from the ground up."
--Stephen Johnson, interview for O'Reilly Network

"Emergence is the process of complex pattern formation from more basic constituent parts or behaviors, and manifests itself as an emergent property of the relationships between those elements."
--Wikipedia

• Cities and self-organization.



• Ant Colonies: Social Organization Without a Director?



• Software

• Braitenberg's Vehicles [ definition ]
• MIT Epistemology and Learning Group: Exploring Emergence
• Craig Reynolds' "Steering Behaviors For Autonomous Characters"

• Cellular Automata @ Wikipedia
• Cellular Automata @ Math World

• Lights Out Game