This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
A spell is a card on the stack, and only one on the stack. In
other words, a card becomes a spell as you cast it, and stops being one
when it resolves. Note that when playing a land, it doesn't go onto the
stack; lands can't ever become spells.
 
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