Meteorology. a line drawn on a weather map or chart that connects points at which the barometric pressure is the same.
Also, isobare [ahy-suh-bair] . Physics, Chemistry. one of two or more atoms having equal atomic weights but different atomic numbers.
Compare isotope.

1860–65; < Greek isobarḗs of equal weight. See iso-, baro-

isobarism, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
isobar (ˈaɪsəʊˌbɑː)
1.  a line on a map connecting places of equal atmospheric pressure, usually reduced to sea level for purposes of comparison, at a given time or period
2.  physics Compare isotope any of two or more atoms that have the same mass number but different atomic numbers: tin-115 and indium-115 are isobars
[C19: from Greek isobarēs of equal weight, from iso- + baros weight]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1864, coined from Gk. isos "equal," + baros "weight," from barys "heavy" (see grave (adj.)).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

isobar i·so·bar (ī'sə-bär')

  1. Any of two or more kinds of atoms having the same atomic mass but different atomic numbers.

  2. A line on a weather map connecting points of equal atmospheric pressure.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
isobar   (ī'sə-bär')  Pronunciation Key 
A line drawn on a weather map connecting places having the same atmospheric pressure. The distance between isobars indicates the barometric gradient (the degree of change in atmospheric pressure) across the region shown on the map. When the lines are close together, a strong pressure gradient is indicated, creating conditions for strong winds. When the lines are far apart, a weak pressure gradient is indicated and calm weather is forecast.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Isotherms are lines connecting areas of the same temperature, and isobars
  connect regions of the same atmospheric pressure.
On weather maps, locations of equal pressure are connected by isobars,
  typically drawn as white lines.
The forecasted wind vectors there go across the isobars at a much larger angle
  than they should.
Below is an animation of infrared imagery of the storm, with isobars and the
  track of the surface low superimposed.
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