Mendelian randomization (MR) has been increasingly used to interrogate the causal effects of modifiable risk factors on human health and disease over the last 15 years. As MR becomes more commonplace in clinical guidelines and drug development, there is a need for researchers and practitioners from multiple disciplines to understand the existing and rapidly evolving ‘language’ of MR. The MR Dictionary provides useful definitions and descriptions for undertaking, understanding and interpreting MR studies to a wide, inter-disciplinary audience – both those new to MR and those who are experienced in its use but who want to remain up to date. The full searchable (with terms cross-referenced throughout) version of the MR Dictionary is provided on the journal website. In the first instance, we aim to update the MR Dictionary annually and encourage users of the Dictionary to help us in this endeavour. By the first update (August 2020), we aim to have moved to an open, collaborative and interactive online tool on the journal website that will support more rapid and extensive linked searches and more immediate updates and online discussion.