Multivortex Tornado- A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.
Multiple vortices usually occur in groups of 2 to 5 at once and usually last less than a minute each. Tornado scientists now believe that most reports of several tornadoes at once, from news accounts and early 20th century tornado tales, actually were multivortex tornadoes.
See also: What is the meaning of Report, Multiple, Circulation, Vortex, Tornado?