Ok, I can't believe this thread keeps popping up and has gone on for 16 pages, because its such a simple answer and anyone who doesn't know the answer immediately, certainly shouldn't be engaging in any odds-based business, particularly trading.
The definition of gambling is, "to stake or risk money, or anything of value, on the outcome of something involving chance."
http://dictionary.reference.com/browse/gambling
So, in any odds based business or game, whether its poker, blackjack, craps, running a casino, running an insurance company, or trading, there are always two sides to the bet or trade; one side is gambling and one side is playing the odds. Whether you're "gambling" or "playing the odds" depends on what the actual percentages are of your success in that given situation. If you're chances of success are greater than 50%, then you're not gambling, because the odds are in your favor that you'll be successful more than half the time and you'd have to get "unlucky" in order to lose on that particular proposition. If you're on the side where your odds of success are less than 50%, then that means you're gambling because you'd have to beat the odds and "get lucky" in order to get paid on that particular odds based event.
Look at the relationship between the casino and the craps player. No matter what bet the craps player makes, he is always gambling because there isn't any bet he can make where his odds of success are greater than 50%. The casino on the other hand is simply playing the odds, not gambling, because no matter what bet they take the other side of, the odds are in their favor to win any given bet. Does that mean that the casino will always win every single bet and the craps player will never win any bet. Of course not, it just means that as the number of times they take that bet approaches a very large number (remember, the laws of averages are insignificant unless you have a large data sample), their percentage of success (or failure) will move closer and closer to the actual expected odds.
The same relationship exists in poker... If a player moves all-in with the best hand, and is called by a player with an inferior hand, then the player with the inferior hand must get lucky in order to beat the odds and win the hand. The player with the best hand however, only needs to avoid getting unlucky in order to win. Therefore, the player with the worse hand is gambling while the player with the best hand is simply acting like a casino and playing the odds.
Now in the regards to the poker player, he has no way of knowing in advance whether or not he has the best hand before he makes the call. Therefore, even though he may suspect that he's playing the odds, he won't know if he's correct until after he's made the decision to call and is able to see his opponent's hand. If he calls and finds out that his opponent has a better hand than he does, now he's the one who must beat the odds and get lucky in order to win, therefore even though he called because he suspected he was playing the odds, in retrospect he is actually gambling and his opponent is the one who is playing the odds. So if you play poker long enough, you will eventually find yourself in some situations where you are gambling, and you will find yourself in some situations in which you're playing the odds. Depending on the skill of the player, and the length of time he plays the game (hours, days, months, years) he will either find himself in more situations where he's gambling (in which case he'll lose money in the long run) or he will find himself in more situations where he is playing the odds (in which case he will make money in the long run).
So the answer as to whether or not trading is gambling is simple.... it depends. If you're consistently putting yourself in situations where the odds are in your favor, then you're not gambling, rather, you're simply acting like a casino and playing the odds. However, if you're consistently putting yourself in situations where your odds of success are not in your favor, then yes, you're gambling because in order for that trade to work out you have to overcome the laws of averages. And just like the poker player (or the insurance company for that matter), quite often you won't know whether or not you happen to be gambling or playing the odds on any given trade until after the fact, and therefore, whether or not you are consistently gambling or whether or not you're consistently playing the odds will depend upon your skill level as a trader. However, if you take enough trades, you're invariably going to find yourself in some trades where you are gambling and you're going to find yourself in trades where you're not gambling. But over a long enough period of time, good traders will take more trades where they're playing the odds and less trades where they're gambling, and losing traders will take more trades where they're gambling and less trades where they're playing the odds.
So the correct answer is sometimes trading is gambling and sometimes its playing the odds... it just depends on how good you are as a trader at finding situations where the odds are in your favor. However, if you trade long enough and take enough trades, invariably, you'll find that in some trades you ended up gambling and in some trades the person on the other side of the trade was the one gambling.