Volume

Volume is the total number of shares traded during a given period, usually one day. It counts activity, not direction — a day with 10 million shares traded had more participation than a day with 500,000, regardless of whether the price went up or down.

High volume during a price move suggests conviction. If a stock drops 5% on heavy volume, a lot of people were selling. If it drops 5% on light volume, it may have been a handful of trades and the move is less convincing.

Volume tends to spike around earnings reports, news events, and options expiration dates. A sudden jump in volume without an obvious catalyst is sometimes worth investigating — someone may know something, or a large institution may be building a position.