I am an undergraduate student in Electrical Engineering, and I presented my research as a poster at a reputable IEEE conference. In addition to this, my 2-page extended abstract was fully peer-reviewed (quoting the conference site) and published in the proceedings as a 2-page paper. Would I be able to count this as a publication on my CV? Is being peer-review and published in the proceedings the only criteria for something to be counted as a publication?
Yes, it is a publication and should be included in the CV. You would be wise, of course, to mark it as an extended abstract.
You may want to publish a longer version as well, so marking the abstract as such will cause less confusion when/if that happens.