From 'Sitting Buddha' by R.M. Daishin Morgan
Sitting Buddha is another way of speaking about Buddha Nature or the nature of enlightenment. We usually conceive of living the Buddhist life as a path going from delusion to enlightenment, as is implied by referring to it as the Way. In the Zen tradition, there is another view which stems from the understanding that Buddha Nature is not subject to coming and going; it is not born and does not die. Buddha Nature, therefore, is not something we acquire through training. The spiritual life in Zen is realizing Buddha Nature in the present moment. Because all beings are enlightened from the very beginning, this way of understanding Buddha Nature is called original enlightenment. We have nothing missing, we cannot be separate from the truth - realizing this is not so simple.

"Sitting Buddha"