This is a typical technical term used by one of the schools of Kāśmīr Śaivism.
Based on the Śivasūtras of Vasugupta (9th century A. D.), Kalhaṭa or Kallaṭa (A. D. 854) wrote a small work of 51 verses called Spandakārikā which expounds the theory of spanda.
Śiva is the supreme Lord. When his śakti or inherent power starts the process of creation, with the very first wave or vibration, it is called spanda. The 36 evolutes are manifested out of this.