A contrivance is a gadget or device that can be used for some particular purpose. The spinning blade of a blender is a contrivance that turns fruits and yogurt into a tasty smoothie. Yum.
A contrivance is a useful device or tool. It might be assembled or invented, and it's often mechanical, although you can really refer to anything that's made for a specific purpose as a contrivance. The word sometimes also implies a falseness or obviousness, like the contrivance of a spy's very fake-looking beard that's supposed to disguise him, or the contrivance of a novel's complicated, unbelievable plot.