Shiva Purana
विवर्तनाद्ब्रह्मणस्तु ब्रह्मवैवर्तमुच्यते ॥ लिंगस्य चरितोक्तत्वात्पुराणं लिंगमुच्यते ॥ १३२ ॥
Since the transformation of Brahman is narrated the Purāṇa is called Brahmavaivarta. Since the story of Liṅga is mentioned it is called Liṅga Purāṇa.
english translation
vivartanAdbrahmaNastu brahmavaivartamucyate ॥ liMgasya caritoktatvAtpurANaM liMgamucyate ॥ 132 ॥
hk transliteration by Sanscript