Spontaneous acts are not planned or arranged, but are done because someone suddenly wants to do them. Diana's house was crowded with happy people whose spontaneous outbursts of song were accompanied by lively music + spontaneously spon·ta·neous·ly As soon as the tremor passed, many people spontaneously arose and cheered
{s} instinctive, natural; not premeditated, impromptu, unplanned; arising from internal forces (of a natural phenomena)