What is the meaning of short sell?

a. Buy shares from a brokerage firm and enter into a forward transaction to sell the shares at todays price on a future date. b. Buy shares from one brokerage firm and sell the shares to another brokerage firm with a view to make quick money. c. Borrow shares from a brokerage and sell the shares at todays price with the intention to repay the borrowed stock they sold at some future time when the stock is trading for a lower price. d. Sell the shares at todays price to an investor and enter into an option agreement to buy the shares back in near future.