It is not a keyword it is a label.
Usage:
label1: for (; ; ) { label2: for (; ; ) { if (condition1) { // break outer loop break label1; } if (condition2) { // break inner loop break label2; } if (condition3) { // break inner loop break; } } } [Documentation][1]. [1]: http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.7