Divide the sky into twenty-seven equal parts and something is left over. The Moon actually takes about 27.3 days to complete its circuit against the fixed stars, close enough to twenty-seven that a twenty-seven-fold scheme of lunar mansions became standard, but not exact enough to close cleanly, and Vedic astronomers named the discrepancy rather than ignore it. Abhijit, identified with the bright star Vega, is the answer: a twenty-eighth nakshatra, inserted between Uttara Ashadha and Shravana to absorb the leftover arc.
Its credentials are old. The Nakshatra Sukta, preserved in the Taittiriya Brahmana and the Taittiriya Samhita, invokes Abhijit by name among the lunar mansions, connecting it there with Brahma's victory and with the winning of light and prosperity. But its geometry is awkward from the start: because the other twenty-seven divide the ecliptic into neat 13 degree 20 minute segments, giving Abhijit its own equivalent slice means borrowing degrees from both of its neighbors rather than adding a clean extra piece to the circle.
That awkwardness had consequences. As Jyotisha's technical apparatus, the Vimshottari dasha sequence and the gana and nakshatra-lord classifications built on a clean twenty-seven-fold cycle grew more elaborate over the centuries, Abhijit had no natural place in most of it, and later practice largely dropped it from everyday use even while its scriptural mention was never in doubt. It survives today mainly as a historical and muhurta-related curiosity, a nakshatra named in the oldest layer of the tradition and excluded from most of what the tradition built afterward.