A price area where selling has previously stopped an advance and may do so again.
Support, resistance and the flip. Support is a price where buyers keep stepping in and the fall stops; resistance is a price where sellers keep stepping in and the rise stops. Once price closes above an old ceiling, that same level often acts as the new floor.
Resistance is the mirror of support: prior highs, a declining moving-average, round numbers, or an area where trapped buyers want to get out at breakeven.
A breakout above resistance that holds turns the level into support. A breakout that fails is a fakeout or bull-trap.
Example: an index has stalled between 4,780 and 4,800 four times. Buyers who chase into that zone are buying where sellers have consistently shown up.