From The Collaborative International Dictionary of English v.0.48:
Binder \Bind"er\, n.
1. One who binds; as, a binder of sheaves; one whose trade is
to bind; as, a binder of books.
[1913 Webster]
2. Anything that binds, as a fillet, cord, rope, or band; a
bandage; -- esp. the principal piece of timber intended to
bind together any building.
[1913 Webster]
3. a pair of stiff oblong covers, sometimes detachable,
designed for insertion of paper pages to create a
book-like document, such as in a loose-leaf binder.
[PJC]