Śvetāśvatara Upaniṣad · 2..14
य इमं त्र्यक्षरं निखिलं ब्रह्मोपासते द्यावापृथिव्योरन्तरेण यत् तत् ते श्वेतं श्वेतम्
ya imaṃ tryakṣaraṃ nikhilaṃ brahmopāsate dyāvāpṛthivyorantareṇa yat tat te śvetaṃ śvetam
Those who worship this complete trisyllable, Brahman, what is between heaven and earth —that is for you white, white.
The tryakṣara (trisyllable) is again Oṃ, and here it is described as nikhila (complete, total). Brahman is what permeates all space between dyau (heaven) and pṛthivī (earth), that is, the entire manifested universe. In our yoga practice, upāsanā (worship/meditation) is not external ritual but interior contemplation. The repeated “white” suggests absolute purity, absence of stain or mixture. One who thus contemplates sees the entire cosmos as expression of that pure Reality.