A sound produced by a particular musical instrument is usually called a tone.
A sound of a single frequency is called a monotone. Thus, a monotone means a pure sound of one single frequency.
We use a tuning fork to produce a monotone.
In fact, a tuning fork is the only device that can produce a sound of one single frequency.
A tuning fork is a U-shaped metallic device that consists of a handle (stem) and two tines (prongs). The two prongs are joined with the stem and form a U-shape.
When one of the prongs of the tuning fork is gently hit with a rubber hammer (or pad), the prongs start vibrating. The back and forth vibrations of the prongs produce a sound.