'A'a: Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments.

A.D.: anno Domini, used to record the number of years after the death of Christ

Andesite: a gray, fine-grained volcanic rock

Basalt: A hard, dense, dark volcanic rock composed chiefly of plagioclase, pyroxene, and olivine, and often having a glassy appearance.

B.C.: Before Christ, used to record the number of years before the birth of Christ

B.C.E.: Before Common Era; formally known as B.C.

C.E.: Common Era; formally known as A.D.

Composite cones (stratovolcanos): steep-sided, consist of layers of both pyroclastic material and lava. Rocks comprising composite cones range from silica-poor varieties (basalt) through intermediate types to silica-rich types,
ex: Cascade Range of California, Oregon, Washington, and British Columbia.

Convergent: tending to come together from different directions

Debris: what's left of something destroyed

Deformation: Change of shape by pressure or stress.

Divergent: tending to move apart in different directions

Eruption: A violent throwing out of flames, lava, etc., from a volcano

Infrared: Invisible radiation wavelengths just below the color red in the electromagnetic spectrum.

Geothermal: Internal heat of the earth.

Hotspot: a spot inside the mantle that heats areas of the plate above it

Lava: magma that reached the surface

Lahar: A landslide or mudflow of volcanic remains

Magma: a mixture of finely divided solids with enough liquid to produce a pasty mass

Mantle: The layer of the earth between the crust and the core; a solid body of rock in between the molten core and thin crust at the surface.

Pahoehoe: Lava with a smooth glassy surface.

Plates: One part of the earth's layers that moves constantly.

Pyroclastic: Composed chiefly of rock fragments of volcanic origin

Pyroclastic Cones (cinder cones): small, steep-sided, composed largely of pyroclasts of various sizes with little, if any, lava., Occur in groups and are associated either with major volcanoes in arcs or with isolated, local volcanic terranes,
ex: Wizard Island at Crater Lake; San Francisco Mountain volcanic field, Arizona

Shield cones: low rounded profiles (2-10°), cone-shaped accumulations of lava containing minor amounts of interlayered pyroclastic materials,
ex: Snake River Plain in western North America

Stratovolcano: A volcano made of changing layers of ash and lava.

Subduction: When two plates meet and one slides beneath the other

Tephra: Solid matter that is ejected into the air by an erupting volcano

Terrain: An area of land

Tsunami: A very large ocean wave caused by an underwater earthquake or volcanic eruption.

Viscosity: thickness of a liquid (i.e. molasses)

Volcanic Domes: small, steep-sided, shaped like inverted cups or domes, Formed by the intrusion, extrusion, or both of thick (viscous) siliceous magma, typically associated with major volcanoes, but can also occur in independent fields of intermediate to siliceous volcanic structures that lack a single major volcano.
ex: Long Valley Caldera (Mono Lake area, California)

Volcano: An opening in the earth's crust through which molten lava, ash, and gases are ejected.

Volcanologist: A person who studies volcanoes

Water vapor: Water in a gaseous state, especially when diffused as a vapor in the atmosphere and at a temperature below boiling point.

Definitions courtesy of Dictionary.com, science.ubc.ca