The term "bull" has several meanings depending on its context.
1) In stock market, a bull refers to an investor who believes that the market is going up and is optimistic about future growth.
2) In sports, particularly in wrestling or boxing, it means a charge made by an animal (usually a bull) towards someone or something with the intention of attacking them.
3) In slang terms, "bull" can also mean nonsense or rubbish - as in "that's just bull."