A gap is a change in price levels between the close and open of two consecutive days. Although most technical analysis manuals define the four types of gap patterns as Common, Breakaway, Continuation and Exhaustion, those labels are applied after the chart pattern is established. That is, the difference between any one type of gap from another is only distinguishable after the stock continues up or down in some fashion. Although those classifications are useful for a longer-term understanding of how a particular stock or sector reacts, they offer little guidance for trading.
A gap is defined as a price level on a chart where no trading occurred. These can occur in all time frames but, for swing trading, we are mostly concerned with the daily chart.
A gap on a daily chart happens when the stock closes at one price but opens the following day at a different price. Why would this happen? This happens because buy or sell orders are placed before the open that cause the price to open higher or lower than the previous day's close.
One thing to point out is that it does not state that the gap has to be a certain percentage or $ value. So this can be subjective to each person in how they look at it.