X-axis - The horizontal axis on a graph.

X-Ray Radiation - A form of electromagnetic radiation with a wavelength between 0.03 and 30 nanometers.

Xenolith - A rock that has become embedded in another rock when it was magma.

Xerophyte - A plant that has adaptations to survive prolonged periods of drought.

Xerosphere - Places and regions on the Earth that are deficient in water. Plants and animals found in these regions have evolutionary adaptations to survive drought.

Xylem - A type of specialized tissue found in vascular plants used to quickly and efficiently transport water and mineral nutrients.