Good point, but an even better definition would take variance and risk of ruin into account. If you just look at expectation, then buying insurance would qualify as a gamble. I think most people would say it's more of a gamble to skip insurance.

Betting all your money on one hand of blackjack is a gamble, even if you've been counting cards and know the odds are in your favor. A casino spreading its money over millions of small bets is investing.