MG/ML is the unit that measures the strength of nicotine present in an E-Liquid. It is the milligram of nicotine present in 1ml of e-liquid. So an 18mg/ml nicotine composition of E-Liquid will have 18 milligrams of nicotine in 1ml of liquid. Standard nicotine composition in the UK is 0mg/ml, 3mg/ml, 6mg/ml, 18mg/ml and 20-25mg/ml