An action of selling or trading shares with an expectation of the price will go down over time and can rebuy at a lower price to gain profit.
A short, or short position, is a directional trading or investment strategy where the investor sells shares of
borrowed stock in the open market. The expectation of the investor is that the price of the stock will decrease
over time.
Read more: Short (or Short Position) Definition | Investopedia http://www.investopedia.com/terms/s/short.asp#ixzz4UcU6i1bq
What I find interesting is that some people refer to shorting to not only selling and expecting to buy back cheaper, but doing so with leverage. As in shorting implies leverage (which means they are losing more money than the difference between the selling point and the new higher price after selling). You can end up in debt even.
I think most people reefer to that when they talk about shorting, which is why it's so dangerous, you can go bankrupt if you screw it up. A lot of people were aiming to short the market at 1k expecting a drop and now they are losing their shit, look at the graph.. it's 1030 already and climbing.
Never mind the current price right now or even yesterday - you have an important point to take note of. A lot of traders are using incredible leveraging to short and while this sounds attractive, it simply puts you at so much exposure to sudden spikes in opposite directions.
Learn to trade first with small or no leverage, and always with a lot of healthy room for your stop loss to activate.