A whale is a big holder of crypto, and usually have a substantial amount of capital. Basically, they’re rich.

This is a term borrowed from casino gamblers

For Bitcoin, anyone with more than 1,000 BTC is generally considered a whale.

Whales who are bullish (believes prices will rise) on the price of a specific cryptocurrency. These folks are also referred to as bullish whales. 

Whales are often the market movers for small illiquid altcoins due to their huge capital.