eruption

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or process of erupting.
• noun An instance of erupting.
• noun A sudden, often violent outburst.
• noun An appearance of a rash or blemish on the skin.
• noun Such a rash or blemish.
• noun The emergence of a tooth through the gums.

from The Century Dictionary.

• noun A bursting forth; a sudden breaking out, as from inclosure or confinement; a violent emission or outbreak: as, an eruption of flame and lava from a volcano; an eruption of military force; an eruption of ill temper.
• noun The act of forcibly expelling matter from inclosure or confinement.
• noun In pathology: A breaking out, as of a cutaneous disease.
• noun The exanthema accompanying a disease, as the rash of scarlet fever.
• noun Synonyms Outburst, outbreak.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of breaking out or bursting forth
• noun A violent throwing out of flames, lava, etc., as from a volcano or a fissure in the earth's crust.
• noun A sudden and overwhelming hostile movement of armed men from one country to another.
• noun A violent commotion.
• noun That which bursts forth.
• noun A violent exclamation; ejaculation.
• noun (Med.) The breaking out of pimples, or an efflorescence, as in measles, scarlatina, etc.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a violent ejection, such as the spurting out of lava from a volcano
• noun a sudden release of pressure or tension
• noun an infection of the skin resulting in a rash or blemishing

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the emergence of a tooth as it breaks through the gum
• noun symptom consisting of a breaking out and becoming visible
• noun a sudden violent spontaneous occurrence (usually of some undesirable condition)
• noun the sudden occurrence of a violent discharge of steam and volcanic material
• noun (of volcanos) pouring out fumes or lava (or a deposit so formed)
• noun a sudden very loud noise