A breaker is a piece of reef against which waves break.
This sense of the term is closely related to another sense, that of the breaking wave itself. In times past, when ocean navigation was relatively rudimentary, a line of breaking waves was a crucial indicator that a boat was bearing down upon an island or reef. This was especially the case when sailing in darkness, in which the highly visible line of froth caused by breaking waves could be the only such indicator. In such cases the line of breakers was charted and named as a "breaker".