Historical Rules · 1 October 2010
Rule 107.4f
Official source text from the 1 October 2010 Rules release
The snow mana symbol {S} represents one generic mana in a cost. This generic mana can be paid with one mana of any type produced by a snow permanent (see rule 204.4f). Effects that reduce the amount of generic mana you pay don't affect {S} costs. (There is no such thing as "snow mana"; "snow" is not a type of mana.)