I don't suppose for a moment that there is a clear and set definition of a mountain but it's interesting to hear what different folk see or perceive a mountain to be. Guy and Mark both highlight the fairly clear fact that what makes a mountain in the UK might not apply elsewhere, given relative heights of 'mountains' in different countries. I suppose this leads back, to a degree, to the aspect, the look and feel of the peak.
The problems of splitting a mountain range into separate mountains, with all the different criteria applied to different levels - munros, corbetts, grahams, marilyns, hewitts etc - can only underline the reality that there is no clear-cut definition - so what of those personal definitions - other than the obvious one of heght ? What of physical character, climate etc ?