Break of Structure (BOS) is a term from price-action trading for a simple, objective idea: in an uptrend, price makes higher highs and higher lows, and a break above the prior swing high confirms the trend continues. (Mirror for downtrends: a break below the prior swing low.) It's a rules-based way to define trend health without an indicator.
What a BOS confirms
Markets trend by breaking prior structure. A BOS — price taking out the most recent swing high in an uptrend — is objective evidence buyers are still in control and the trend has legs. As long as price keeps making BOS in one direction, the trend is intact. It turns “is this still trending?” into a yes/no you can see, which is why it's useful on a fast intraday chart.
How to use it
Use BOS as a trend-continuation filter: after a BOS, look to enter on the pullback to the broken level or a higher low, in the trend's direction. Each new BOS is a re-confirmation to keep trading with the trend. The failure to make a new BOS — a pullback that instead breaks the prior swing low — is a change of character, warning the trend may be turning.
A break of structure is the trend proving it's alive: each new push takes out the last high. Trade with it until price fails to make one.
The honest frame
BOS is a structural read that describes trend, not a magic signal — and defining “swing” points has some subjectivity. It's most useful combined with the regime (BOS in a negative-gamma trend is high-conviction) and the dealer levels (a BOS through a wall is a strong continuation). Use it to stay on the right side of a trend and to spot when the trend structure breaks down.