I came across a Slideshare presentation from a guy called Umar Ghumman.
It's pretty simple ... about 40 advertising strategists and planners give their definition of an insight. Some are relatively long, which is kind of funny. Others are really simple. All of them are worth reading.
I found this blurb from Martin Weigel, Head of Planning at Weiden + Kennedy (Amsterdam) particularly refreshing ...
Let’s reclaim insight as a way of looking and thinking, and take it off its pedestal of unhealthy attention and worship.