(n.) A line connecting or marking points on the earth's surface having the same temperature. This may be the temperature for a given time of observation, or the mean temperature for a year or other period. Also, a similar line based on the distribution of temperature in the ocean.

air mass anticyclone cold front cold sector cyclone front high high-pressure area isobar isometric isometric line isopiestic line isothermal line low low-pressure area occluded front polar front squall line stationary front warm front weather map wind-shift line