Q. How do I footnote a reference to an online dictionary definition (Oxford English) in a PowerPoint presentation please?
A. In a presentation, it’s best not to distract your audience with a lot of bibliographic data (unless the point is to dissect a source citation). Instead, simply mention the source in your talk or include a brief attribution somewhere on the slide.
Brexit, n. The withdrawal of the United Kingdom from the European Union, and the political process associated with it. Following a referendum held in the UK on 23 June 2016, in which a majority of voters favoured withdrawal from the EU, the European Union (Withdrawal) Act was passed by Parliament on 26 June 2018, as a result of which the UK formally left the EU on 31 January 2020.
—Oxford English Dictionary
It’s always a good idea also to include a final slide (or slides) that list sources in full. There you can include an expanded citation, shown here in the style of an unnumbered note:
OED Online, “Brexit, n.,” accessed June 11, 2026, https://doi.org/10.1093/OED/1134583781.
You don’t have to discuss this final slide (you don’t even have to show it), but it serves as a detailed record that documents your research and stays with your presentation.