Three-Quarters of Brahman's Realm
Tripāda-vibhūti ('the three-footed/three-quarter manifestation') is the term from the Puruṣa Sūkta: pādosya viśvā bhūtāni tripādasyāmṛtam divi — 'All beings are one quarter of Him; three-quarters are the immortal in heaven.' This describes Brahman's totality as divided between the transcendent eternal realm (nityavibhūti — three-quarters) and the material manifestation (līlāvibhūti — one-quarter).
The Eternal Realm
The tripāda-vibhūti is the nityavibhūti — the eternal, unchanging realm of Vaikuṇṭha consisting of pure consciousness (śuddhasattva). It is 'three-quarters' in the sense that it vastly exceeds the material creation — the universe, for all its apparent vastness, is but a tiny fraction of Brahman's totality. The eternal realm is where Brahman is fully, freely, and completely Himself.
The One-Quarter
The eka-pāda-vibhūti (one-quarter realm) is the līlāvibhūti — the material universe that Brahman creates, sustains, and dissolves as cosmic play (līlā). This includes the entire physical cosmos as well as all the subtle and causal realms of conditioned existence.
Importance of the Ratio
The 3:1 ratio is theologically significant: it establishes that saṃsāra, for all its weight and pervasiveness in our experience, is the minor part of reality. The eternal, the pure, and the blissful vastly predominate. This prevents a world-negating or despairing attitude: saṃsāra is real but not the whole of reality.