The term Dasha, Maha Dasha, or Vimshottari in Hindu Vedic Astrology is used to indicate planetary periods.  The planetary periods indicate when the good or bad effects are produced according to their placement by sign, start, house, combinations and aspects.

In Hindu Vedic Astrology, it is assumed that the maximum duration of a native is 120 years.  These 120 years are further divided into 9 major parts called MahaDasha or Vimshottari dasha ruled by particular planets, like Ketu 7 yrs, Venus 20 yrs, Sun 6 yrs, Moon 10 yrs, Mars 7 yrs, Rahu 18 yrs, Jupiter 16 yrs, Saturn 19 yrs and Mercury 17 yrs.  Further these 9 major periods are further divided into sub-periods, sub-sub-periods and so on.

This system is used in almost all astrological techniques in India and this is given greatest importance.