This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
It's a marker on the stack to represent an ability. When a spell
is played, the spell card itself gets put onto the stack. However, when an
ability of a permanent is played there isn't an appropriate card handy- so
what gets put on the stack instead is a pseudospell, as a marker.
 
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