[ahy-sab-nawr-muh l]
noun Meteorology.
  1. a line on a map or chart connecting points having an equal deviation from the normal value of some meteorological quantity, as temperature.
Compare isanomal.

Origin of isabnormal

First recorded in 1850–55; is- + abnormal Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018