Citation, Documentation of Sources
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 (proposed) withdrawal of the United Kingdom from the European Union, and the political process associated with it. Sometimes used specifically with reference to the referendum held in the UK on 23 June 2016, in which a majority of voters favoured withdrawal from the EU.
—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, s.v. “Brexit, n.,” accessed May 2, 2019, http://www.oed.com/view/Entry/54763375.
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.