Gap Up / Down
When a session opens above or below its previous close, leaving an unfilled area on the chart.
In forex, weekend gaps are the main case - if a major event happens Saturday, Sunday's open can gap from Friday's close. Gaps tend to get filled (price returns through them) in the following sessions, which is why some strategies fade them.
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Learn to actually use Gap Up / Down.
Definitions are the easy part. The free first five modules put this on a real chart and make you do the work. No card required.