The concept of "enrichment" started among people who care for captive wild animals (zoos, laboratories, animal sanctuaries, and wildlife rehabs). They wanted to improve the welfare of animals under their care, but they noticed occasional signs of distress — stereotypies (highly repetitive, functionless behaviour, such as repetitive pacing, swaying, head-bobbing, licking, bar-biting, and dirt eating), horse stereotypies (cribbing/windsucking, weaving, wood chewing, stall kicking), self-harm (excessive licking, feather-plucking, overgrooming), and other abnormal behaviors.

(WARNING — Some of the videos in the above paragraph may be disturbing, as some show animals in very obvious psychological distress)

It was discovered that one way to reduce these signs of distress was to provide various forms of mental stimulation. Although any form of mental stimulation helps, the goal is usually to encourage behavior observed in that species outside of captivity. Some examples:

• food enrichment — encouraging foraging behavior, instead of providing all the animal's food in one easily-accessed place
• physical habitat enrichment — enlarging an animal's enclosure, or providing toys and other enclosure features that encourage play
• social enrichment — housing multiple animals of the same species together, to encourage social interaction (and even mixed-species groups, if they're able to get along)

At this point, the concept of enrichment goes far beyond eliminating overt signs of stress, and is now used to try to improve animal welfare overall, for even apparently well-adjusted captive animals, both wild and tame (pets, and sometimes farm animals).

Other introductory articles:

• UntamedScience.com, and one of their YouTube videos
• Sara Web at the Tennessee Aquarium (YouTube)