What is the definition of a wheelchair under the ADA?

A wheelchair is a manually operated or power-driven device designed primarily for use by an individual with a mobility disability for the main purpose of indoor, or of both indoor and outdoor, locomotion.  Individuals with mobility disabilities must be permitted to use wheelchairs and manually powered mobility aids, i.e., walkers, crutches, canes, braces, or other similar devices designed for use by individuals with mobility disabilities, in any areas open to pedestrian traffic.

For additional information, take a look at the following resources: 

Fact Sheet: The ADA & Accessible Ground Transportation

Power Driven Mobility Devices

Fact Sheet: Wheelchairs and Other Power-Driven Mobility Devices