To what does the term "pyroclastic" refer?

The word "pyroclastic" simply means something that is formed by or involving fragmentation as a result of volcanic or igneous action. It comes from the Greek “πῦρ” (“fire”) and “κλαστός” (“broken”). It is most commonly found in two expressions.

Pyroclastic flow is a fluidised mixture of hot rock fragments, hot gases, and entrapped air that moves at high speed in thick, dark grey, turbulent clouds that hug the ground. The temperature of the volcanic gases can reach about 600 to 700 °C (1,100 to 1,300 °F). The velocity of a flow often exceeds 100 km (60 miles) per hour and may attain speeds as great as 160 km (100 miles) per hour. Flows may even travel some distance uphill when they have sufficient velocity, which they achieve either through the simple effects of gravity or from the force of a blast out of the side of an exploding volcano. Reaching such temperatures and velocities, pyroclastic flows can be extremely dangerous.

Pyroclastic rocks are composed of fragments produced and ejected by explosive volcanic eruptions. They are deposits made predominantly of volcanic particles. Some are formed from volcanic steam explosions; others from explosive interaction of magma with groundwater. One of the most spectacular types of pyroclastic deposit is an ignimbrite, which is the deposit of a rapidly flowing hot suspension of pyroclasts in gas. Ignimbrites may be loose deposits or solid rock, and they can bury entire landscapes.