Usability

in general usability means the following

• the system is easy to use and re-use
• the system can be used efficiency
• small amount of bugs / failures
• users are happy with software

but think also about factors like

1. Effectiveness - customer can find anything they like to.
2. Efficiency - data can be find easily and fast.
3. Satisfaction - customers are satisfied of your application and they come again and again

this has been translated from the following website: http://www.samplezone.ch/ideologie/usability/ I absolutly aggree with above terms if they really do and life this terms I don't know.