Data, pre- "Generations", famously claims that he cannot feel emotion. However, the exact definition of "emotion" is not made clear in the series, as far as I know. Clearly he feels curious, and has often referred to things as "fascinating". Both of these could be called "emotion".

Is there anything written, either in stories/film or manuals for the writing staff, that clearly define what Data can and cannot do in terms of expressing emotion?