any of a genus (Solenopsis) of ants whose sting causes a burning sensation, esp. a South American species (S. invicta) that has become a severe pest in the S U.S.
See fire ant in American Heritage Dictionary 4
noun
Any of several ants of the genus Solenopsis, especially S. geminata or S. saevissima of the southern United States and tropical America, that build large mounds and can inflict a painful sting.