noun, plural volcanoes, volcanos.
a vent in the earth's crust through which lava, steam, ashes, etc., are expelled, either continuously or at irregular intervals.
a mountain or hill, usually having a cuplike crater at the summit, formed around such a vent from the ash and lava expelled through it.

1605–15; < Italian < Latin Volcānus, variant of Vulcānus Vulcan

caldera, fumarole, geyser, volcano.
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World English Dictionary
volcano (vɒlˈkeɪnəʊ)
n , pl -noes, -nos
1.  an opening in the earth's crust from which molten lava, rock fragments, ashes, dust, and gases are ejected from below the earth's surface
2.  a mountain formed from volcanic material ejected from a vent in a central crater
[C17: from Italian, from Latin VolcānusVulcan1, whose forges were believed to be responsible for volcanic rumblings]

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Word Origin & History

1610s, from It. vulcano "burning mountain," from L. Vulcanus "Vulcan," Roman god of fire, also "fire, flames, volcano" (see Vulcan). The name was first applied to Mt. Etna by the Romans, who believed it was the forge of Vulcan.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
volcano  [%PREMIUM_LINK%]     (vŏl-kā'nō)  Pronunciation Key 
  1. An opening in the Earth's crust from which lava, ash, and hot gases flow or are ejected during an eruption.

  2. A usually cone-shaped mountain formed by the materials issuing from such an opening. Volcanoes are usually associated with plate boundaries but can also occur within the interior areas of a tectonic plate. Their shape is directly related to the type of magma that flows from them—the more viscous the magma, the steeper the sides of the volcano. ◇ A volcano composed of gently sloping sheets of basaltic lava from successive volcanic eruptions is called a shield volcano. The lava flows associated with shield volcanos, such as Mauna Loa, on Hawaii, are very fluid. ◇ A volcano composed of steep, alternating layers of lava and pyroclastic materials, including ash, is called a stratovolcano. Stratovolcanos are associated with relatively viscous lava and with explosive eruptions. They are the most common form of large continental volcanos. Mount Vesuvius, Mount Fuji, and Mount St. Helens are stratovolcanos. Also called composite volcano. See more at hot spot, island arc, tectonic boundary, volcanic arc.

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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

volcano definition

A cone-shaped mountain or hill created by molten material that rises from the interior of the Earth to the surface.

Note: Volcanoes tend to occur along the edges of tectonic plates.
Note: Eruptions and lava flows associated with them can be very destructive. (See Mount Saint Helens and Mount Vesuvius.)
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
The scientists now believe that volcanoes can produce two kinds of lightning
  during an eruption.
To a lesser extent, they also come from natural processes such as volcanoes and
  forest fires.
Moreover volcanoes spewing ash into the atmosphere can have effects on the
  climate too.
But it is tiny, and wrinkled, the lava of long-exhausted volcanoes furrowing
  and bending the landscape this way and that.
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