The term is using in BTC which means a big capital, big money Bitcoin players that show their hand in the Bitcoin market. Has been applied to the wrong class of investor because the players described below are truly the biggest creatures. The largest players have to stagger and obscure their market entries by splitting large trades into hundreds or thousands of small orders.
It is not only in BTC as you can be a whale even in an altcoin market. As long as you are big player in any market, it makes it easy for you to control that market to your favor based on the perception you want other traders to have to enter the market. When you see a whale at work, you will always know. All the above, sometimes, unless you are really holding a very large one, most whales always come together to make that possible anyway, so I do not usually see it as a one man thing for a big market.