A spindown life counter is a die that is designed for the express purpose of keeping track of life totals. Its sides are in a sequence such that adjacent numbers are next to each other, facilitating the change of life total and avoiding any complications with finding the desired life total. This is different from a standard 20-sided die, on which the numbers that are opposite of each other on the die add up to 21 (1 is opposite from 20, 2 is opposite from 19, and so on), as is traditional for dice in general.
The "20" on a spindown life counter is replaced with an expansion symbol (or in the case of core sets the appropriate number).
Spindown life counters are generally found in fat packs, beginning with Apocalypse, and are generally made in the five Magic: The Gathering colors (white, blue, black, red, green) for each set. There was a gold-colored spindown life counter in the Alara Reborn fat pack as the set is gold based.