युगप्रजालक्षणम् ऋषिप्रवरवर्णनं च
Yuga–Prajā-Lakṣaṇa and the Enumeration of Eminent Ṛṣis
यः कश्चित्पादवान्मध्ये प्रयुक्तो ऽक्षर संपदा / विनियुक्तावसानां तु तामृचं परिचक्षते
yaḥ kaścitpādavānmadhye prayukto 'kṣara saṃpadā / viniyuktāvasānāṃ tu tāmṛcaṃ paricakṣate
A mantra that has pādas, is properly set in the middle with fullness of syllables, and ends as prescribed—this is called a Ṛk.