
1) Atmospheric dynamic 2) Having the same density 
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 Isopycnic means `of the same density.` In particular, an isopycnic surface is a surface of constant density. This term is a bit more obscure than the similar terms isobaric or isothermal surfaces, which describe surfaces of constant pressure and constant temperature respectively. It is common in conversational use to hear isopycnic surfaces refe.....
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http://en.wikipedia.org/wiki/Isopycnic

line connecting points of equal density
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• (a.) Having equal density, as different regions of a medium; passing through points at which the density is equal; as, an isopycnic line or surface. • (n.) A line or surface passing through those points in a medium, at which the density is the same.
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http://thinkexist.com/dictionary/meaning/isopycnic/

<physics> Having equal density, as different regions of a medium. ... A line or surface passing through those points in a medium, at which the density is the same as, an isopycnic line or surface. ... Origin: Iso- + Gr. Pyknos dense. ... (08 Mar 2000) ... 
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http://www.encyclo.co.uk/local/20973

Having equal density: thus in equilibrium density gradient centrifugation a particle (molecule) will cease to move when it reaches a level at which it is isopycnic with the medium.
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 I`so·pyc'nic  adjective
I`so·pyc'nic  adjective [ 
 Iso- + Greek 
 pykno`s dense.] 
 (Physics) Having equal density, as different regions of a medium; passing through points at which the density is equal; as, an 
 isopycnic line or surface. 
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http://www.encyclo.co.uk/webster/I/103

having the same density.
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https://www.infoplease.com/dictionary/isopycnic
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