The importance of the phenomenon of temporary labour migration is growing both from a quantitative and from a political point of view. Its legal regulation is grounded on concepts, such as “temporariness” and “labour market”, which are either not defined by the law or in potential conflict with their socio-economic reality. We propose two approaches to better identify the meaning of these terms in the context of European Union (EU) law, by focusing on the three categories of labour migration which are explicitly defined as temporary. These are posting of workers, seasonal work, and intra-corporate transfers. We analyse the concept of temporariness through a necessary and sufficient conditions approach, whereas to identify the meaning of “labour market” law we propose an original systematic document analysis using a corpus of more than 1800 mentions of the concept in EU “hard law”. Our article highlights the polysemic nature of these concepts and proposes operationalised criteria for further research into the legal regulation of temporary labour migration in the EU.