Plagiarism and academic Teaching use Often while teaching (Engineering) it is convenient to illustrate a concept by a figure / photo / sketch etc. which is pre-existing in literature: perhaps another book, or vendor document or a code or a presentation etc. Now, of course it is possible in theory to attribute every such instance and I do where I can.
But I was just wondering if a lack of attribution would be considered Plagiarism in this context?
My point is that most definitions of Plagiarism (e.g. see below a definition from Wikipedia) have a crucial element of &quot;representation of another author's as one's own original work&quot;

Plagiarism is the representation of another author's language,
thoughts, ideas, or expressions as one's own original work

Assuming I am abundantly clear that the presentation or talk is NOT original work (to the extent that I can add a disclaimer on every presentation and maybe each slide) doesn't that obviate any issues with Plagiarism?
Another definition of Plagiarism lists out five defining elements one of which is:

....In a situation in which there is a legitimate expectation of
original authorship

Would classroom teaching (to undergrads) be a situation where there's is a legitimate expectation of original authorship?
More generally, this is not about stealing anyone's credit but about ease of informal discourse. e.g. If in every conversation I had with people if I had to mention the source of every idea or point I was making it would add a lot of drudgery.
To rephrase: In informal context like teaching notes or slides how essential is it to attribute every sketch, drawing, photo etc.
Often it is implicit that the work is not original. e.g if I am comparing a feature of two bridge designs using photos there's no way anyone could even confuse them to be my original work. In many cases the original creator is not clear and could be found out but would take additional digging around.
I just imagine there's a tradeoff between scrupulous attribution versus clarity of communication and efficiency of creating teaching content.
PS. I suppose copyright should not be an issue due to the &quot;Classroom Use Exemption&quot;
Would classroom teaching (to undergrads) be a situation where there's is a legitimate expectation of original authorship?

No, but that is irrelevant to plagiarism.

if a lack of attribution would be considered Plagiarism in this context?

In any academic context, attribution is required.  So yes.