Definition of 'Bull'
An investor who thinks the market, a specific security or an industry will rise. Investors who takes a bull approach will purchase securities under the assumption that they can be sold later at a higher price.
A "bear" is considered to be the opposite of a bull. Bear investors believe that the value of a specific security or an industry is likely to decline in the future.
Investopedia explains 'Bull'
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. Bears, in comparison, 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 real estate or commodities, like soy beans, crude oil or even peanuts.