= Thermal equilibrium
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Two internally equilibrated systems are in thermal equilibrium if bringing them into contact that permits only <heat> exchange causes no net <heat transfer>. In ordinary nonrelativistic <thermodynamics> without appreciable gravitational temperature shifts, their <thermodynamic temperatures> are equal. Merely preventing <heat transfer> with insulation does not establish <thermal equilibrium>: two insulated bodies can have different <temperatures>. <University Physics Volume 2 by Samuel J. Ling and others/1.1 Temperature and Thermal Equilibrium>[Samuel J. Ling, William Moebs and Jeff Sanny, OpenStax University Physics Volume 2, section 1.1]{ref}
A <thermometer> illustrates the definition. Its own <temperature> settles to the object's <temperature> after thermal contact, provided its effect on the object is negligible. The existing <zeroth law of thermodynamics> makes this comparison transitive: two objects separately in <thermal equilibrium> with the same <thermometer> are in <thermal equilibrium> with each other. This lets one calibrated scale compare different materials.
Equal <temperatures> do not imply equal <internal energies> or equal <heat capacities>. Nor does <thermal equilibrium> by itself imply <mechanical equilibrium> or <chemical equilibrium>; these require additional allowed exchanges. There is also a distinction between no net exchange in <thermal equilibrium> and a constant <temperature> maintained by equal incoming and outgoing <heat>. The latter can be a <steady state> with ongoing <entropy production>. Microscopic exchanges can continue at <thermal equilibrium>, but their opposing contributions balance on average.
Historically, OpenStax credits the name “zeroth law” to Ralph Fowler in the 1930s: the already numbered laws needed a logically earlier statement about <thermal equilibrium>. <University Physics Volume 2 by Samuel J. Ling and others/1.1 Temperature and Thermal Equilibrium>[Ling, Moebs and Sanny, section 1.1: naming the zeroth law]{ref}
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