I'm sure this has been asked before but I cannot find in anywhere. Say you are in the key of C major. What is the name of a dominant chord on the 6th scale degree, i.e. A C# E G. This device is ubiquitous in pop music, and off the top of my head 'Fallen Down' from the Undertale soundtrack is the first simple tune that comes to mind which features it. I have been mentally referring to this as a Neapolitan chord but this is a different harmonic device.
Often it'll be a secondary dominant or V/V, V/ii, V/iii, V/vi. In other words, it's non-diatonic (containing C#), and often leads, in this case, to Dm. It's the V of Dm, - A7. It can be named VI7, but that doesn't give much of a clue as to what it's actually doing in relation to other chords around it. But if it doesn't lead to Dm (ii), then it's more of a standalone, and can be labelled VI7.
So to sum up, in key C major, it's called, in RN, V7/ii. Or simply, A dominant 7.