Muṇḍaka Upaniṣad · 2.1.2

दिव्यो ह्यमूर्तः पुरुषः सबाह्याभ्यन्तरो ह्यजः । अप्राणो ह्यमनाः शुभ्रो ह्यक्षरात्परतः परः

divyo hy amūrtaḥ puruṣaḥ sabāhyābhyantaro hy ajaḥ | aprāṇo hy amanāḥ śubhro hy akṣarāt parataḥ paraḥ

Divine, incorporeal is the Person, both external and internal, unborn; without breath, without mind, pure, beyond the Immutable, the supreme.

Description of the transcendent Person through negations and attributes.

Divyaḥ — divine, celestial, resplendent. Not earthly, not limited by the laws of matter.

Amūrtaḥ — incorporeal, formless. Not subject to spatial limitations.

Sabāhyābhyantaraḥ — both external and internal. Beyond the interior/exterior duality; it is the field in which everything happens.

Ajaḥ — unborn, uncaused. Without beginning, without cause, self-existent.

Aprāṇaḥ amanāḥ — without breath, without mind. Beyond vital and mental functions; the source of both.

Śubhraḥ — pure, bright, white. Free from all contamination, luminous by nature.

Akṣarāt parataḥ paraḥ — beyond the Immutable, the supreme. Even the Akṣara (the manifested Immutable) is surpassed by this absolute Person.

This is the description of Brahman nirguṇa, without qualities, beyond all conception.