A historically significant price level that price action has had a difficult time trading above.

If price action rise and approaches a confirmed resistance level, a re-test of resistance occurs and it can be either confirmed again or broken.

A test at resistance is confirmed if a bearish change in the market sentiment occurs at the resistance level. 

Resistance is broken when the bullish market sentiment does not change at the resistance level and price action continues up through the resistance level.

Resistance ones broken became support.