For example, a presentation whose PDF is listed at http://www.astro.ex.ac.uk/exoclimes/2012/pdf/talks/Day02_Ferreira.pdf?
And what if the presentation doesn't have a publicly available URL? How would the citation style differ from that of a poster?
It turns out that one commonly used weight update strategies for neural networks, RMSProp, was first introduced in a slide:
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Given the number of citations, it should give you plenty of examples on how to cite a slide.
One common citation format:
T. Tieleman and G. Hinton. Lecture 6.5-rmsprop: Divide the gradient by a running average of its recent magnitude. COURSERA: Neural Networks for Machine Learning, 4, 2012.