The opposite is a design that forces the user to stretch, to make extra effort, in order to interact successfully with the interface or device.
Usability is a measure of the success of the design in supporting or enhancing human performance.
FDA Human Factors Program - a web site about the ways that medical devices intended to save, prolong, and improve the quality of life can, in some cases, fail to provide the benefit they are designed for, and in the more unfortunate cases, cause injury or death. Human Factors Principles for Medical Device Labeling - labeling in this instance consists of directions on how to use and care for medical devices as well as risks, precautions, warnings, potential adverse reactions. The report available at this site presents the principles for writing the labels for medical devices. Usability Toolkit - a collection of forms, checklists and other useful documents for conducting usability tests and user interviews. Usability Quiz - Test your knowledge of basic usability principles.