Transformer Winding Resistance
There's the primary DC resistance, and the secondary DC resistance.
In addition, there's an AC resistance for each which will be frequency dependent.  This is not the impedance due to the magnetizing or leakage inductance, it's a true lossy resistance due to the skin effect, proximity effect and core losses.  So it will add to the losses in the transformer. [There's also the concept of mutual resistance or the AC resistance of a winding changing due to the current distribution in an adjacent winding, but it's mostly ignored.]
If you just want the DCR, you measure at DC.  If you want to measure the true AC resistance vs. frequency it's a difficult measurement and is sometimes even done via a calorimeter.