An ergonomic design is one that is ment to work well with the human form -- designed using the principles of ergonomics. It is most often used to refer to furniture (i.e. 'an ergonomic chair') and appliances (particularly computer peripherals, like an ergonomic mouse or a natural keyboard), although clothing, houses, and pretty much anything else might designed with ergonomics in mind.
From erg-, Greek for work + nomic short for economic. It was first created in the mid- to late 1940s. It was originally used to mean the scientific study of the efficiency of people in the workplace, although now it is often thought of more as a matter of improving comfort and health rather than just efficiency.