staple (WIRE)
noun [C] 
a short thin piece of wire used to fasten sheets of paper together. It has bent ends which are pushed through the paper and then flattened by a special device 

A staple is also a U-shaped piece of metal with sharp ends which is hammered into a surface to hold something, such as a wire fence, in a particular position. 

A staple-gun is a tool which you hold in your hand and use to push staples into a surface. 

staple
verb [T] 
Would you mind stapling the reports together?

stapler
noun [C] 
A stapler is a small device which you can hold in your hand or use on a table to push staples through pieces of paper.