This word is used with various different meanings, and when it is used it should be defined. In thermodynamics 'adiabatic' is used in a macroscopic sense to refer to a process occurring in a thermally insulated system, so that there is no flow of heat to or from the surroundings. In @R05170@, the word has been used in a microscopic sense, with a range of meanings which have only a tenuous relationship to the thermodynamic meaning or the etymology. Whereas the thermodynamic meaning relates to conditions imposed on a process by an observer, the microscopic meaning relates to conditions under which the process occurs naturally. The microscopic meanings, as used in @R05170@, all have in common the feature that quantum states remain unchanged during the course of reaction. Different quantum states may be referred to: A reaction that is not adiabatic is referred to as nonadiabatic or diabatic, and some workers make a distinction between the two words.

1. A reaction in which there is no change of @ET07026@ or multiplicity has been called adiabatic, or more specifically electronically adiabatic.
2. A reaction in which there is no change of vibrational state during the course of reaction has been said to be vibrationally adiabatic. More loosely, the expression has been applied to a process in which excess vibrational energy in the @R05163@ appears as vibrational energy in the @P04861@, or in which ground-state vibration in the reactants leads to @G02704@ vibration in the products.
3. A reaction in which excess rotational energy in the reactants appears as rotational energy in the products, or in which ground-state rotation in the reactants leads to ground-state rotation in the products, has been referred to as rotationally adiabatic.
4. In the @R05390@ theory of @M03989@ reactions, a degree of freedom whose quantum number is more or less preserved during energization and subsequent reaction has been called 'adiabatic'; the word 'inactive' has also been applied to it.