Interestingly, I would venture to say that DPI is a measure of resolution: that's the way it's still used in printers or scanners, for example (600 dpi). And retina instead means high angular resolution, or pixels having small arc measures from "appropriate" distances.

The term "resolution" transitioned gradually to mean number-of-individual-lines-rendered horizontally and vertically for displays, but really, the idea is how many dots can you "resolve" (or "resolving power"): a "high resolution" screen or screen mode had a larger number of individual pixels being drawn, which meant that the density is higher. You never talk about a scanner having a resolution of 7200x3600 even if it can scan 12"x6" at 600dpi.

So really, in an informal conversation, I believe both are fine. If you want to be extremely precise, you missed the mark: width and height in pixels is the sanest way to call what you refer to as "resolution".