An adjustable spanner or adjustable wrench is a spanner

Wrench

A wrench or spanner is a tool used to provide grip and mechanical advantage in applying torque to turn objects—usually rotary fasteners, such as nuts and bolts—or keep them from turning....

 with a "jaw" of adjustable width, allowing it to be used with different sizes of fastener

Fastener

A fastener is a hardware device that mechanically joins or affixes two or more objects together.Fasteners can also be used to close a container such as a bag, a box, or an envelope; or they may involve keeping together the sides of an opening of flexible material, attaching a lid to a container,...

 head (nut

Nut (hardware)

A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...

, bolt

Screw

A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...

, etc.) rather than just one faster, as with a conventional fixed spanner. An adjustable spanner may also be called a shifting spanner, shifter, fit-all, or adjustable angle-head wrench.
In many European countries (eg France, Germany, Portugal, Spain and Italy) the adjustable spanner is called an "English key" as it was first invented in 1842 by the English engineer Richard Clyburn.