From Gk. adiabatos "impassable," from a- "not" +
diabatos "passable," from diabainein "to go across,"
from dia- " through" + bainein "to go."

Bidarrow from bi- "not; without" + darrow "way out,"
from dar "out" + row "to go, going," from raftan
"to go."

adiabatic change

دگرشد ِ بیدررو، دگرش ِ ~

degaršod-e bidarrow, degareš-e ~

Fr.: changement adiabatique

A change taking place in a system that has perfect thermal insulation,
so that heat cannot enter or leave the system and energy can only
be transferred by work.

The assumption whereby the density fluctuations in the very
→ early Universe would be produced by
compressing or decompressing of all components of a homogeneous Universe.
The adiabatic initial conditions lead to coherent oscillations in the form of
peaks in the → temperature anisotropy spectrum.
See also → acoustic peak,
→ baryon acoustic oscillation.

The temperature gradient defining the → radiative equilibrium
condition in a region. It is expressed as:
dT/dr = (1 - 1/ γ)((T / P)(dP / dr),
where T and P are temperature and pressure, dT / dr and
dP / dr temperature and
pressure gradients respectively, and γ = C_{P} / C_{V}.
For radiative equilibrium to be stable against → convection,
the actual temperature gradient must be less than the adiabatic temperature gradient, i.e.
|dT /dr|_{rad} < |dT /dr|_{ad}.
See also → Schwarzschild's criterion.

A condition in which there is an excess of the actual temperature gradient
over the → adiabatic temperature gradient
corresponding to the same pressure gradient.
A region with superadiabatic temperature gradient is convectively unstable.
→ Hayashi forbidden zone.