So, to be somewhat formal about it, I went to dictionary.com and looked up "gambling":
<i>to stake or risk money, or anything of value, on the outcome of something involving chance</i>
Going by that alone, I'd have to consider any money risked in a financial market a gamble (trading, investing, etc), with the possible exception of some forms of arbitrage where the outcome is basically guaranteed. The definition is a slippery slope, because in the "unbounded" sense, you could claim the element of "chance" in just about anything and, thus, call many potential decisions gambles (eg: the decision to commute to work today, let alone the decision to pursue career X).
But, obviously, most people aren't going to claim the income they earn from their careers on the "gambling" line of their tax returns, and that goes for traders and investors, as well. So, at least at tax time, the definition narrows a bit.
What isn't mentioned in that definition is whether the expectancy is fixed to favor one of the two wagerers (thinking slot machines, most casino table games), or whether the expectancy can shift over time between the wagerers due to learning, skill, etc (poker, trading). When you think about a casino, we tend to think of the "gamblers" as the casino's customers... not the casino itself... because in most games, the casino's expectancy is fixed positive (exception might be some blackjack games w/ skilled players, etc). For example, I'm going to assume that casinos don't report their income from slot machines and table games on the "gambling" line on their tax returns (hence the notion that if your expectancy is well-known to be positive, at least the IRS might not consider it gambling).
My guess is most of these points might have already been mentioned somewhere in the previous 160+ pages of this thread, so I apologize if I haven't said anything new.