उपासनं तु यथाशास्त्रसमर्थितं किञ्चिदालम्बनमुपादाय तस्मिन् समानचित्तवृत्तिसन्तानकरणं तद्विलक्षणप्रत्ययान्तरितम् … ।
upāsanaṁ tu yathā-śāstra-samarthitaṁ kiñcid ālambanam upādāya tasmin samāna-citta-vṛtti-santāna-karaṇaṁ tad-vilakṣaṇa-pratyayāntaritam … |
(Śaṅkarācārya’s bhāṣya on Chāndogya Upaniṣād: 1.1.1)
“Upāsanā means taking up some basis [i.e., object of meditation] as established by śāstra and maintaining a continuous flow of a singular course of thought (citta-vṛtti) [in regard to it] that is protected from thought distinct from it.”
Commentary
Simply put, upāsanā is defined here as an uninterrupted, one-pointed stream of thought focused on a specific object of worship that is enjoined in śāstra.