What is Bull Spread?

Legal Definition
In options trading, a bull spread is a bullish, vertical spread options strategy that is designed to profit from a moderate rise in the price of the underlying security.

Because of put-call parity, a bull spread can be constructed using either put options or call options. If constructed using calls, it is a bull call spread. If constructed using puts, it is a bull put spread.
-- Wikipedia
Legal Definition
In options trading, a bull spread is a bullish, vertical spread options strategy that is designed to profit from a moderate rise in the price of the underlying security.