Ionizing radiation is a broad category that includes portions of the EM spectrum. EM radiation is ionizing if it has enough energy to remove at least one electron from an atom, this includes the upper bits of UV and above. There are things other than EM that are things that are ionizing radiation too like subatomic particles.

Geiger counters can also detect XRays more or less effectively depending on their design.

> Ionizing radiation is any type of particle or electromagnetic wave that carries enough energy to ionize or remove electrons from an atom. There are two types of electromagnetic waves that can ionize atoms: X-rays and gamma-rays, and sometimes they have the same energy.

http://physicscentral.com/explore/action/radiationandhumans....