An option strategy whereby a number of put options are bought at a certain exercise price and an equal number of puts (with same expiration date and underlying asset) are sold at a higher exercise price. This bullish strategy produces its profits when the underlying asset price increases. And so, it is the reverse of a bear call spread, that is, it works similarly but in the opposite direction. The bull put spread technically belongs to the family of vertical spread because it involves options of the same stock, same expiration month, but with different exercise prices.
Bull put spreads are useful for investors who are fairly optimistic on an underlying stock, i.e. if they expect the stock price to go up moderately, so to profit from such a bullish market. Thanks to the put-call parity, this strategy can be replicated by taking two opposite positions on the same call option in a similar strategy known as a bull call spread.
The bull put spread is also called a short put spread.