Through an extensive review of primary and secondary sources – including legal documents, peer-reviewed articles, books, and grey literature across disciplines – we explore what the term “girl child marriage” means and why it more accurately captures current global efforts than other terms like early, teenage, or adolescent marriage. The lack of harmonised terminology – particularly in global public health – prevents alignment amongst different stakeholders in understanding what the problem is, in order to determine how to measure it and create solutions to address it.