An investor who will profit from a fall in the price of a security without owning it has a short position in it.
Short selling is a way of profiting from a decline in a price by selling a security without owning it. This may be done by borrowing a holding of the security.
Short positions may form part of many different strategies. Some may aim to profit from an expected decline in a market, others may use short positions to hedge a downside risk rather than profit from it. A particular example of the latter are market neutral porfolios that are completely hedged against market movements.