How to cite a University lecture (or its slides or script)

Quoting from the biblatex documentation, section 2.1.1:

unpublished: A work with an author and a title which has not been formally published, such as a manuscript or the script of a talk.

Thus, you could use unpublished instead of misc.

I see, however, a general problem in referring to something that has not been published: the reader has no possibility to check what you are referring to, which is, in my humble opinion, the whole point in making references.

To address some commentaries on the accepted answer: The university of Canterbury differences in their APA style between retreivable (as in powerpoint slides) and non retrievable lecture notes (as in notes, someone took during lecture), the latter being defined as personal communication.

If retreivable, it depends where to retreive it. If it is published online, I would use the @online or indeed @misc as most answers citing webpages use that. To add the link to the retreivable resource use an url entry and be sure to load for example the hyperref package. If it is published elsewhere, you cite that source (@book etc.).

For the case of notes taken by yourself or not taken at all, it falls under the category of personal communication and you have to decide wether to include the reference in the bibliography at all (the link supposes not to) or rather use inline citation. Further reading for personal communication from the APA blog here.

For me, I often use something like this. We need the URL (howpublished) to the slides, and the note to indicate the last access.

@misc{slides_on_ABC,
  author={Joe and John},
  title = {Title of the slides},
  howpublished = {\url{https://www.abc.xyz/the_slides.pdf}},
  year = {2018},
  note = {Accessed: {2020–11-06}}
}