A period of trading activity that occurs bewteen 4am-Opening Bell (9:30am). However, your brokerage may not offer pre-market trading until as late as 8am. Institutional Investors have full access at 4am.
"Many investors and traders watch the pre-market trading activity to judge the strength and direction of the market in anticipation for the regular trading session. Pre-market trading activity generally has limited volume and liquidity, and therefore, large bid-ask spreads are common. Many retail brokers offer pre-market trading, but may limit the types of orders that can be used during the pre-market period."
( http://www.investopedia.com/terms/p/premarket.asp#ixzz3fUjhTIuq )
