A breaker block is an order block that failed — price traded through it instead of respecting it — and is then expected to act as support or resistance from the opposite side on a retest. The mechanical reading: take the last opposite candle before a move that swept a swing and then broke structure the other way; once price breaks back through that origin, its old block becomes a breaker.
The concept, from ICT and smart-money trading, applies the polarity principle to order blocks: a level that fails as one role can flip to the other. A bullish breaker forms when a down-move’s order block is violated to the upside after a structure shift, and price returning to it is treated as support; a bearish breaker is the mirror. The story is that trapped traders from the failed move defend the level as they manage losses.
This is among the more discretionary ICT constructs — it stacks several already-variable definitions (order block, sweep, structure shift) and teachers disagree on each — so a breaker-block rule is only testable once every one of those components is fixed numerically. Definitions vary, and any specific recipe is a hypothesis to backtest, not a settled standard.