isobront

Century Dictionary and Cyclopedia

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

Wiktionary

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

GNU Webster's 1913

1. n. (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.