It's mg/ml, but generally simply labeled as mg.
--Deeper down the rabbit hole of labeling information below--
The weight of pure nicotine is 1001mg/ml, so if you want to convert the mg/ml label to weight by volume, simply move the decimal point one place to the left. 15mg/ml = 1.5% w/v, 25mg/ml = 2.5% w/v, 75mg/ml = 7.5% w/v, etc.