milli is a prefix used in the metric measurement system used in most countries around the world.
milli = 1/1000 (thousandth)
eg A millimetre is 1/1000 of a metre - a metre broken into 1,000 equal parts.
eg A millilitre is 1/1000 of a litre - a litre of liquid broken into 1,000 equal parts.
eg A milligram is 1/1000 of a gram - a gram broken into 1,000 equal parts.
1 metre (m) = 1,000 millimetres (mm)
1 litre = 1,000 milliltres (mL)
1 gram (g) = 1,000 milligrams (mg)