Functions and Features
In software parlance, quite a lot of time, we mix up functions and features. I feel they are unique. In my opinion, Functions are those usecases that can be achieved from the software or application. Features are the external views of functions - the way a function may be exposed.
For example, 'Saving a file' is a 'Function'. This can be achieved by multiple features: From Menu bar (Alt+F+S), through Shortcut (Ctrl+S), through Toolbar (click the 'Disk' icon).
Also, I think fundamentally software should look at the functional requirements and the Usability Experts should work around the functional requirements to create the features that are usable. Many a time, I've seen both getting mixed up. Sometimes one sees things like 'ability to send a mail' as a 'feature' and 'checkbox to enable notification mail' as a 'function'.
Function:Feature = 1:Many, Feature:Function = 1:1 (is that true?)