http://en.wikipedia.org/wiki/Nicotine#Toxicology
Website 1)
"Firstly, the strength description: “54mg”, indicates a nicotine concentration of 54mg (milligrams) per ml (millilitre) and not per bottle. This equates to a strength of approximately 5.4% w/v (weight/volume) (i.e. dependent on specific gravity there are approximately 1000mg in 1ml.) It is important to understand this, and that it is the relative proportion of Platinum Ice in your mixed juice, and not the absolute quantity, that will determine final strength. For example; If you dilute Platinum Ice with a nicotine free dilutant, such as Propolyne Glycol, to a 50:50 ratio by volume then the resulting liquid will contain 27mg/ml."
Website 2)
"We're often asked exactly what 12mg or 24mg strengths mean for nicotine.
In short, it's the amount of nicotine in milligrams, for each millilitre of E-Liquid, expressed as mg/ml. Some companies choose to represent this as a percent per solution, which is where you'll see 1.2% instead of 12mg/ml.
This means the same thing, just interpreted differently.
Also one of the most common questions about an E-cig relates to which e-liquid nicotine strength is equivalent to a particular kind of cigarette that a person smokes.
To better answer this question, it is better to understand how much nicotine is contained in the E-Liquid you are inhaling.
The nicotine density listed for e-liquid is the number of milligrams of nicotine per milliliter in the E-Liquid. That means a 10ml bottle of 24mg nicotine e-liquid would have 240mg of nicotine in it (24mg x 10ml). Likewise, a 30ml bottle of 24mg e-liquid would have 720mg of nicotine in it (24mg x 30ml)."