What is Bull?

Legal Definition
A Bull is a stock market speculator who buys a holding in a stock in the expectation that in the very short-term it will rise in value whereupon he will sell the stock to make a quick profit on the transaction. Strictly speaking the term applies to speculators who borrow money to fund such a purchase, and are thus under great pressure to complete the transaction before the loan is repayable or the seller of the stock demands payment on settlement day for delivery of the bargain. If the value of the stock falls contrary to his expectation, a bull suffers a loss, frequently very large if he is trading on margin. A bull has a great incentive to "talk-up" the value of his stock or to manipulate the market in his stock, for example by spreading false rumour, to procure a buyer or to cause a temporary price increase which will provide him with the selling opportunity and profit he requires. A bull must therefore be contrasted with an investor, who purchases a stock in expectation of a medium-term (say 5 years) or long-term increase in value due to the underlying performance of the company and its assets. The speculator who takes a directly opposite view to the bull is the bear, who speculates on a stock decreasing in value, having sold short. A bull market is a period during which stock market prices rise over a sustained period, therefore to the advantage of bulls.
-- Wikipedia
Legal Definition
Eccles. law. A letter from the pope of Rome, written on parchment, to which is attached a leaden seal, impressed with the images of Saint Peter and Saint Paul.

2. There are three kinds of apostolical rescripts, the brief, the signature, and the bull, which last is most commonly used in legal matters. Bulls may be compared to the edicts and letters-patent of secular princes: when the bull grants a favor, the seal is attached by means of silken strings; and when to direct execution to be performed, with flax cords. Bulls are written in Latin, in a round and Gothic hand. Ayl. Par. 132; Ayl. Pand. 21; Mer. Rep. h. t.
-- Bouviers Law Dictionary
Legal Definition
In ecclesiastical law. An instrument granted by the pope of Rome, and sealed with a seal of lead, containing some decree, commandment or other public act, emanating from the pontiff. Bull, in this sense, corresponds with edict or letters patent from other governments. Cowell; 4 Bl. Comm. 110; 4 Steph. Comm. 177, 179. This is also a cant term of the Stock Exchange, meaning one who speculates for a rise in the market.
-- Black's Law Dictionary
Legal Definition
A papal edict.
-- Ballentine's Law Dictionary