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A whale is crypto slang for a person or group that owns so cryptocurrency that they can influence the market and essentially the price of that cryptocurrency.
In the beginning of Bitcoin and many other cryptocurrencies the price was so low that it was easy for the early adopters to acquire a large quantity of those cryptocurrencies.
And today there are groups and individuals that holds thousands of Bitcoins and use that to influence the price and take advantage of this to buy at a lower price or selling at a higher price.
Whales are often used for memes in the crypto world.