An investor who thinks the market, a specific security or an industry will rise.
Bulls are optimistic investors who are presently predicting good things for the market, and are attempting to profit from this upward movement. For example if you are bullish on the S&P 500 you will attempt to profit from a rise in the index by going long on it. Bulls are are the exact opposite of the market's bears, who are pessimistic and believe that a particular security, commodity or entity will suffer a decline in price.
Bullishness does not necessarily apply only to the stock market; you could for example be bullish on just about anything, including commodities like soy beans, crude oil or even peanuts.