A few regular dictionaries don't provide a definition of a bear, some define a bull in stock market terms.
Bull: "speculate for the rise" - Concise Oxford Dictionary.
Bear: "speculator who sells stocks or shares that he may or may not possess because he expects a fall in prices" - Dictionary of Economics (Penguin)
Some of the more elegant and interesting definitions are found in the first English dictionary composed and published by Samuel Johnson in 1755:
Pension: "An allowance made to any one without the equivalent. In England it is generally understood to mean pay given to a state hireling for treason to his country."
Probably means bad policy advice.
Stockjobber: "A low wretch who gets money by buying and selling shares in the funds."
Excise: "A hateful tax levied upon commodities, and adjudged not by the common judges of property, but by wretches hired by those to whom excise is paid."
Bull: "A stock-jobber."
Bear: "A description of stock-jobbers, who sell unreal stock."
Johnson's last one is a gem and two other definitions of bears are included on the following page.