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

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.