A steep-sided mass of lava that is generally formed immediately above the volcanic vent from which it was extruded.  Domes are generally circular in plan and have a relatively small surface area relative to other types of lava flows.  Domes may be spiny, rounded, or flat on top, and often have rough, blocky surfaces formed by the fragmentation of the dome’s crust during intrusion.  Domes may grow by extrusion of lava onto the outer surface of a previously formed dome (exogenous dome) or may be formed by inflation of a pre-existing dome (endogenous dome).  Domes are most commonly the result of extrusion of viscous lava (primarily of the composition of rhyolite and dacite, but andesite may occur as well).

Glossary of Volcanic Terms - G. J. Hudak, University of Wisconsin Oshkosh, 2001

Courtesy of http://www.indiana.edu/~sierra/papers/2003/Phillips.html