What is the term for an adjustable spanner with the opening at the side?
It's a monkey wrench, if you can believe that, or a coach wrench. 


  Adjustable coach wrenches for the odd-sized nuts of wagon wheels were manufactured in England and exported to North America in the late eighteenth and early nineteenth centuries. They were set either by sliding a wedge, or later by twisting the handle, which turned a screw, narrowing or widening the jaws. In 1840, Worcester, Massachusetts knife manufacturer Loring Coes invented a screw-based coach wrench design in which the jaw width was set with a spinning ring fixed under the sliding lower jaw, above the handle. This was patented in 1841 and the tools were advertised and sold in the United States as monkey wrenches, a term which was already in use for the English handle-set coach wrenches.




And no, it wasn't named after a guy with a funny surname.