Whale
A whale is a rich, weak player who gambles with large amounts of money and loses a lot. Other players are happy to see a whale at the table, since he plays loose and loses big pots.
The simple explanation
The name comes from casinos, where a whale is a big spender. In poker, a whale plays many hands, often with marginal holdings, and does not care about the size of the pots.
A closer look
A whale is not just any weak player: he also has a bankroll large enough to continue. Good players try to get a seat at his table, often to the left of him, so they can act after him. Treat whales with respect: kind behavior keeps them at the table, and angle shooting or mocking drives them away. The other terms for weak players are fish and recreational player.
A practical example
A rich businessman plays $25/$50 for fun, calls most bets and loses $40,000 in a night. The regulars call him a whale.
Full explanation
A whale is a rich, weak player who gambles with large amounts of money and loses a lot. Other players are happy to see a whale at the table, since he plays loose and loses big pots.
A whale is a weak player with a lot of money.
He plays many hands and large pots.
Good players want to sit at his table.
Treat him well so that he stays.