Spontaneous
SPONTANEOUS, adjective [Latin , of free will.]
1. Voluntary; acting by its own impulse or will without the incitement of any thing external; acting of its own accord; as spontaneous motion.
2. Produced without being planted, or without human labor; as a spontaneous growth of wood.
SPONTANEOUS combustion, a taking fire of itself. Thus oiled canvas, oiled wool, and many other combustible substances, when suffered to remain for some time in a confined state, suddenly take fire, or undergo spontaneous combustion.