A
wave which needs a
medium in order to propagate itself.
Sound waves, waves in a Slinky, and water waves are all examples of this. Sound waves need air
molecules in order to exist; the
Slinky waves need the Slinky, and the waves in the
ocean need the water.
It follows, then, that mechanical waves cannot exist in a vacuum. This is the factor that distinguishes them from electromagnetic waves.
All Wikipedia text
is available under the
terms of the GNU Free Documentation License