Breakout

What Does Breakout Mean?
A price movement through an identified level of support or resistance, which is usually followed by heavy volume and increased volatility. Traders will buy the underlying asset when the price breaks above a level of resistance and sell when it breaks below support.  

 
This chart shows a stock that has historically encountered a lot of resistance near $37, but notice how it heads sharply higher following the breakout.

Investopedia explains Breakout
In practice, a breakout is most commonly used to refer to a situation where the price breaks above a level of resistance and heads higher, rather than breaking below a level of support and heading lower. Once a resistance level is broken, it is regarded as the next level of support when the asset experiences a pullback  Most traders use chart patterns and other technical tools such as trendlines to identify possible candidates that are likely to break through a support/resistance level. 

A breakout is the bullish counterpart to a breakdown.