Whale is a big player that can move the market and manipulate it according to his needs, it can be a single person or also can be a group of people who collaborate with each other for making profit. Whale definition is vague and can mean different things in different markets, like to manipulate the price of ethereum market you would need to have more than 100 bitcoins at least, but for small cap markets like those coins having market less than $20 million then you can be a whale even with about 20 to 30 bitcoins.