1 [count] : a usually neat pile : a group of things that are put one on top of the other
He had arranged the letters in stacks.
She took a magazine from near the top of the stack.
— often + of
a stack of dishes/papers
2 [count] : a large amount of something — usually + of
There is a stack of evidence against her.
They have stacks of money.
3 [count] : a tall chimney on a factory, ship, etc., for carrying smoke away : SMOKESTACK
4 the stacks : the rows of shelves where books are stored in a library
I couldn't find the book in the stacks.