Saturn in Jyotisha is Shani, literally 'the slow-mover' — the slowest of the nine (about two and a half years per sign), and so it governs time itself: age, slowness, delay, discipline, labour, limitation, the settling of karma. It's also the enforcer of justice: whatever you planted, it comes to collect when due.
Mechanically, Saturn is at home in Capricorn and Aquarius, exalted in Libra and debilitated in Aries. Libra is balance and fairness: Saturn at its best is just discipline. Aries is impulse and immediacy: hurry is what Saturn tolerates least, and there it turns hurry into frustration. The tradition's most dreaded stretch, sade sati, is Saturn's passage around the natal Moon — it takes what you don't need, and the taking hurts.
Where you'll recognise it: things slower than expected, having to endure, heavy responsibility, a feel for structure and rules, life easing as you age. Running well, Saturn is your most reliable part: lasting, bearing, building foundations; achievement comes late and doesn't fall. Stuck, it's rigidity or despair — duty with no life in it, or the sense that it'll never get better. Saturn's lesson is respect for time: what it gives is neither much nor fast, but what it gives is what you actually have.
