isobront

from The Century Dictionary.

• noun A line on a map or chart connecting those places at which a given peal of thunder is heard simultaneously.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A line drawn through geographical points at which a given phase of thunderstorm activity occurred simultaneously.