Bhagavad Gita Chapter 9, Verse 2

राजविद्या राजगुह्यं पवित्रमिदमुत्तमम् | प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम् ||

rāja-vidyā rāja-guhyaṁ pavitram idam uttamam pratyakṣāvagamaṁ dharmyaṁ su-sukhaṁ kartum avyayam

This is royal knowledge (raja vidya), royal secret (raja guhya), pure, supreme, directly realizable, righteous, very easy to perform, and imperishable.

Krishna describes this knowledge with eight qualities that reveal its supreme nature. It's raja vidya (royal knowledge)—the highest knowledge, like a king among all knowledge. It's raja guhya (royal secret)—the most confidential secret, like a king's secret. It's pavitram (pure)—free from material contamination, completely pure. It's uttamam (supreme)—the highest, beyond all other knowledge. It's pratyaksha-avagamam (directly realizable)—you can perceive it directly, not through theory alone. It's dharmyam (righteous)—in accordance with dharma, the path of righteousness. It's su-sukham (very easy)—easy to perform, not difficult. It's avyayam (imperishable)—eternal, never perishes. This knowledge is the highest path—pure, direct, easy, and eternal.