Interesting question, one which I've toyed with myself once or twice.
It's somewhat a subjective definition but it could be generally applied based upon some basic general rules if there was a reason.
But what would be a reason to?
I believe the term 'intercity' was conceived by BR as a marketing policy and was designed to convey the image of speed and comfort. It was also a way of differentiating between what was, at the time, profit making markets and loss making markets (Regional Railways). There was also NSE, but I digress.........
In terms of defintion, you could include stopping pattern but there's alot on traditional intercity routes that call at smaller stations on at least part of a longer route (such as alot of the exGWR routes) and many mixed services that can be a mix of fast/semi-fast and stopping at different stages along a route. Theres also route distance, market type, speed and comfort.
Are there any more worms in this freshly opened can? (I had to include a fishing pun somewhere :P)
Would it also depend the geographical context? Rural vs urban, London based and non London based, country, etc.....