The conjunction “or” can be used not only to indicate two or more alternatives but also to indicate synonymous or equivalent expressions. When used for the latter purpose, “or” can be translated as “also referred to as,” “defined as,” or “in other words,” and sentences containing such an appositional “or” are punctuated differently than sentences in which “or” separates alternatives. Let’s look at some examples.

Infrared spectroscopy or electron paramagnetic resonance spectroscopy can be used to follow the kinetics of radical decay.

Here “or” is used to indicate two alternative techniques—infrared spectroscopy and electron paramagnetic resonance spectroscopy—either of which can be used to follow the decay kinetics.  In this type of sentence, “or” separates the two parts of a compound subject, and no additional punctuation is required. What about the following, apparently similar sentence?

Reflectance difference spectroscopy or reflectance anisotropy spectroscopy is a powerful tool for characterizing the surfaces of crystalline materials.

If “reflectance difference spectroscopy” and “reflectance anisotropy spectroscopy” were two different techniques, the sentence would be syntactically  analogous to the previous one. However, “reflectance difference spectroscopy” and “reflectance anisotropy spectroscopy” are in fact two different names for one technique, so here “or” is being used in the sense of “also referred to as.” When “or” is used in this way, the phrase it introduces is customarily enclosed in commas:

Reflectance difference spectroscopy, or reflectance anisotropy spectroscopy, is a powerful tool for characterizing the surfaces of crystalline materials.

You might argue that the commas are unnecessary because the specialized reader will know that these two terms are synonymous. However, using the customary commas will be helpful to nonspecialized readers, providing, of course, that they’re familiar with this punctuation convention. If you want your meaning to be absolutely clear, you might want to revise to

Reflectance difference spectroscopy, also referred to as reflectance anisotropy spectroscopy, is a powerful tool for characterizing the surfaces of crystalline materials.

Here are some additional examples of correctly punctuated sentences containing an appositional “or”:

  • We used quasielastic, or dynamic, light-scattering spectroscopy to assess the sizes and polydispersities of the particles.
  • The dihydropteroate synthase gene is part of the folic acid synthesis gene, or fas gene, and encodes a trifunctional protein.
  • Binding immunoglobulin protein (BiP), or 78-kDa glucose-regulated protein (GRP-78), is highly induced in hypoxic tumors.
  • We studied the mutarotation, or change in optical rotation accompanying epimerization, of 8-ᴅ-galactopyranose in dilute aqueous buffer (pH 4.3)  at 15°C.
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