Mahadasha
every reader
What the tradition reads
A mahadasha is the major, top level period of the Vimshottari dasha system, the span of years during which one of the nine grahas is read as holding general rulership over the whole of a life, before handing that rulership on to the next planet in the sequence. The nine mahadashas run in a fixed order and for a fixed length each: Ketu seven years, Venus twenty, the Sun six, the Moon ten, Mars seven, Rahu eighteen, Jupiter sixteen, Saturn nineteen and Mercury seventeen, the nine lengths summing to the full one hundred and twenty years of the system. Which mahadasha is running at birth, and how much of it remains, is fixed entirely by the exact degree the Moon had reached within its birth nakshatra, so that two people born under the same nakshatra lord can nonetheless begin their lives at very different points inside that lord's mahadasha. The tradition reads the mahadasha lord's own sign, house, dignity and aspects as setting the broad theme of its whole span, refined in turn by whichever antardasha lord is running within it at any given time.
The story
Maha is the ordinary Sanskrit word for great or major, so mahadasha names quite plainly the great, or main level, period, distinguished from the antardasha, the lesser period running inside it, by scale rather than by any difference of kind; both are drawn from the identical proportional scheme and the identical nine planet order. Wikipedia's account of the tradition credits the sage Parashara with naming a wide range of dasha schemes, some forty two in total, though only two, Vimshottari and Ashtottari, remain in ordinary use, and the same source notes plainly that it is not known when the Vimshottari system was first devised or why the number of years allotted to each planet's mahadasha was fixed as it was. Which mahadasha opens a given life, and how much of it remains, is fixed by the lord of the janma nakshatra, the lunar mansion the Moon occupied at birth, a rule the tradition applies without exception regardless of which alternative dasha scheme a particular lineage might otherwise prefer. Vimshottari's mahadasha remains the term Vedic astrologers reach for by default, so that asking which mahadasha someone is in has become, in practice, shorthand for asking which of the nine grahas is presently understood to govern their life.
Written from
Corpus key glossary:mahadasha.