The American Heritage® Dictionary of the English Language, 4th Edition
n. Any of various nocturnal beetles of the family Lampyridae, characteristically having luminescent chemicals in the posterior tip of the abdomen that produce a flashing light. Also called lightning bug. See Regional Note at lightning bug.
n. A nocturnal, bioluminescent beetle of the family Lampyridae.
the GNU version of the Collaborative International Dictionary of English
n. Any luminous winged insect, esp. luminous beetles of the family Lampyridæ.
The Century Dictionary and Cyclopedia
n. An insect which has the faculty of becoming luminous; a lampyrid or elaterid beetle which emits phosphorescent light from organs in some part of the body.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. nocturnal beetle common in warm regions having luminescent abdominal organs
n. tropical American click beetle having bright luminous spots
Word Usage
"A firefly is a strobe-light which is commonly used for calling in aircraft at night."