In writing the discussion for a manuscript describing the variable bleaching response we observed among colonies of Montipora capitata, I have been thinking about the definition of acclimatization. We observed colonies of the same species with the same symbionts and the same environmental history that either bleached or did not bleach. Could acclimatization play a role in this divergent response? This question has made me think quite a bit about the definition of acclimatization. I think there may actually be two different operational definitions of acclimatization depending on context: first, acclimatization may refer to the change within an individual that allows it to cope with a changing environment. In this context, all organisms are constantly acclimatizing as they maintain homeostasis. The second use of acclimatization refers to a previous experience mediating a future response by inducing some non-genetic change in the individual. I think this is the context in which acclimatization is often used in the coral literature, and it is this sense of acclimatization that I do not think is a plausible explanation for the variability in bleaching response among these colonies. This is because acclimatization in this context is a response induced by the environment, and our bleached and non-bleached corals are immediately adjacent to each other, experiencing the same environment. So, it is not possible that the non-bleached corals were altered by some previous experience that the bleached corals did not also experience (unless they are all older?). Therefore, while the non-bleached colonies may have “acclimatized” (definition 1) as temperatures rose in 2014, allowing them to maintain homeostasis and resist bleaching, and the bleached colonies did not, this variability itself cannot be attributed to past experience, or “acclimatization” (definition 2), and therefore may more likely represent adaptation, or a genetic difference between these individuals.