Definition:

The 80/20 rule states that 80% of the effects generated by a system are centered around 20% of the variables in the system. Design teams can use this rule to their advantage by focusing the bulk of their efforts on the 20%. The rest of the system shouldn't be neglected, but a janitor's closet doesn't need to be as well designed as the stadium seats.

Interaction Design:

In interaction design, this most commonly refers to the two or three most trafficked pages, such as the Facebook news feed or a Google search listing. Within Rackspace's Cloud Apps, the Control Panel mailbox listing is our most commonly accessed page and the design team's focus should reflect that.

User Experience Design:

In user experience design, this rule is represented in common experiences that most customers must do — checking out at the grocery store, syncing an iPhone/iPod or filling up your car at a gas pump. Think about your last experience with a self-checkout kiosk at a grocery store. Did you leave with a smile on your face? Were you delighted? Half-assed efforts are causing severe damage to the UX of buying groceries and shows just how damaging this rule can be.