Technical terms can differ markedly between and within academic disciplines, as most interdisciplinary researchers will be aware. Many scientific terms are ‘pluralistic’ in the sense that they pick out distinct concepts in different contexts, and are sometimes ambiguous between them. Conceptual change, such as the change in the meaning of ‘mass’ post-Einstein, can likewise occur for a variety of reasons. Conceptual change and ambiguity are not just practical concerns however, they speak to philosophical questions about scientific realism and notions of scientific law and natural kinds that often lurk in the background of more applied scientific work.

The 2006 (re)definition of ‘planet’ by the International Astronomical Union, which saw Pluto reclassified as a dwarf planet, is one simple case of ambiguity and change. Though simple, I argue that it also serves as an example of a related phenomenon of concealed polysemy, whereby what appears to be a single kind term conceals two or more distinct concepts with independent scientific utility. This tells us something interesting about scientific concepts, scientific progress, and the metaphysics of science. I explain the historical background to this case as a template for pluralistic approaches to conceptual disputes within science and elsewhere.