In Zen Buddhism, keisaku (Japanese; kyosaku in the Soto school) is an attempt by a sensei to alert students to their mindlessness in zazen (sitting meditation), usually administered by a stick. An English translation is stick of compassion.
In a more colloquial fashion, a keisaku is a «reality check», or something that alerts one to further contemplate their situation, in order to understand and work things out.
In Soto Zen, the Keisaku is always administered at the request of the meditator, by way of bowing one’s head and putting the palms together in gassho, and then exposing each shoulder to be struck in turn. In Rinzai Zen, the stick may be used at the discretion of the Ino, the one in charge of the meditation hall. It is not a punishment, but a means to reinvigorate and awaken the meditator who may be tired from many sessions of zazen.