" is a term used to describe the condition when a stock opens at a higher price than it closed the prior day. The word gap refers to the space that is left in the daily chart. It is the "empty space" from yesterday's close to today's open. Gaps can be either up or down and they can happen to all stocks and in all stock markets.
Gaps are measured from the prior day's closing price to the current day's opening price. The post market activity and pre market activity do not affect the gap. Stocks can trade after market hours, and at pre market starting, but these are not considered "normal" market hours.