Shiva Purana
साधनं मूलमन्त्रस्य पुरश्चरणमुच्यते ॥ पुरतश्चरणीयत्वाद्विनियोगाख्यकर्मणः ॥ ९ ॥
The means of chanting the root mantra is called Purascaraṇa. Because of the steps to be taken in front of the action called viniyoga.
english translation
sAdhanaM mUlamantrasya purazcaraNamucyate ॥ puratazcaraNIyatvAdviniyogAkhyakarmaNaH ॥ 9 ॥
hk transliteration by Sanscript