Source : Webster's Revised Unabridged Dictionary (1913)

Isobront \I"so*bront\ ([imac]"s[-o]*br[o^]nt), n. [Iso- + Gr.
   bronth` thunder.] (Meteor.)
   An imaginary line, or a line on a chart, marking the
   simultaneous development of a thunderstorm, as noted by
   observing the time when the thunder is heard at different
   places.