Scientific writings and policy documents define the terms nanomaterial and nanoparticle in various ways. This variation is considered problematic because the absence of a shared definition is understood as potentially hindering nanomaterial knowledge production and regulation. Another view is that the existence of a shared definition may itself cause problems, as rigid definitions arguably exclude important aspects of the studied phenomena. The aim of this paper is to inform this state of disagreement by providing analytical concepts for a systematic understanding (...) of how, and even whether, nanomaterial and nanoparticle could and should be defined. To do this, we review definitions of nanomaterial and nanoparticle presented in research articles and policy documents. Definitions were identified by first conducting a Scopus search and then tracing cited definitions back to their sources. In total, 36 definitions were identified. Theories of definition from philosophy and linguistics provide analytical guidance for structuring and categorizing the identified definitions, and the main analytical dimensions of the definitions are then identified and discussed. Finally, we propose a framework for understanding the process of defining nanomaterial and nanoparticle. This framework considers both the generality needed for a shared understanding and the level of precision required for different purposes. (shrink)