In science, earth-bound refers to processes or reactions that occur on or near the surface of the earth and under standard natural conditions, i.e. standard temperature and pressure (STP), can be approximated as being described as being a constant temperature (isothermal) constant pressure (isobaric process). [1]

Overview
In 1923, American physical chemist Gilbert Lewis described earth-bound reactions as such:

Earth-bound reactions are the more common cases of reactions which run freely, like the combustion of fuel, or the action of an acid upon a metal. Here we consider [isothermal] systems subject to no external forces except a constant pressure exerted by the atmosphere.”

The no external forces criterion given by Lewis refers to systems coupled in some way to an external conjugate force factor, the prime example being those systems harnessed for the production of useful work, e.g. a galvanic cell reaction in a battery, the electrodes of which being connected to an external mechanical motor or other electrical system.

Lewis, to note, did not actually used the term 'earth-bound' but rather used the term 'isothermal constant pressure'. In modern parlance, the former, more intuitive term (earth-bound), is assumed equivalent to the latter, more cumbersome term (isothermal constant pressure). The term 'earth-bound' derives from the fact that reactions and processes that occur on the surface of the earth, e.g. people walking in a park, reactions in a beaker, etc., occur at constant pressure and temperature, which are reaction "conditions" that yield the the Lewis equality (dG = 0) condition for equilibrium, one of the eight standard types of conditions of the varieties of possible equilibrium states processes: [2]

Conditions
NameCriterion for Equilibrium

Constant volumeIsochoric dE – TdS = 0
Constant pressureIsobaric dH – TdS = 0
Constant temperatureIsothermal d(E – TS) + PdV = 0
Constant entropyIsentropicdE + PdV = 0
Constant volume and entropyIsochoric-isentropicdE = 0
Constant pressure and entropyIsobaric-isentropicdH = 0
Constant volume and pressureIsochoric-isobaricd(E – TS) = 0
Constant pressure and temperatureIsobaric-isothermal
(earth-bound)
d(E + PV – TS) = 0
d(H – TS) = 0
dG = 0


In sum, the term ‘earth-bound’ is short for ‘constant temperature (isothermal) constant pressure (isobaric process)’, which is a rather unwieldy term. In earth-bound processes and reactions, the Lewis inequality:

ΔG < 0

becomes the criterion for spontaneity and the ‘naturalness’ of processes.

Notes
A few conditional factors should be studied or noted in more advanced analysis. Firstly, as has traditionally been the case, for earth-bound processes ‘inside’ of an animated entity, such as a bacteria or human, this criterion almost always holds, on the logic that body temperature is almost always maintained as uniform, 98°F for a human, and pressure is generally assumed to be that of the atmosphere. For earth-bound processes occurring ‘between’ animated entities, such as reactions between people, such as in love or war, this criterion can be approximated to hold, on the basis that atmospheric pressure is generally constant (1atm), implying that:

dP = 0

and temperature generally fluctuates ±10° F on any given day, which can be approximated to imply that:

dT = 0

These are the simplifications used in basic calculations. In more advanced calculations, wherein one is looking to quantify more subtle aspects of temperature and pressure, however, one would need to investigate what effects daily or hourly fluctuations in temperature and pressure would have in the calculations under study. An example might be how the differences in summer temperatures as compared to winter temperatures, affect free energy calculations, as well as how a daily variation in temperature would affect calculations. Beyond this, in very advanced calculations, one would need to investigate how such things as ‘social pressure’ or ‘sexual temperature’ play into the calculation.

See also
Earth-bound thermodynamic system

References
1. Lewis, Gilbert N. and Randall, Merle. (1923). Thermodynamics and the Free Energy of Chemical Substances (pg. 160). McGraw-Hill Book Co., Inc.
2. Rossini, Frederick D. (1950). Chemical Thermodynamics (pg. 123). John Wiley & Sons, Inc.

External links
Standard conditions for temperature and pressure (STP) – Wikipedia.

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