Millilitre (mL)
A millilitre is a measure of liquid volume — one thousandth of a litre, and the same volume as one cubic centimetre.
Millilitres describe how much liquid there is, and say nothing at all about what is dissolved in it. A millilitre of plain water and a millilitre of a concentrated solution occupy identical space.
On a U-100 insulin syringe, one millilitre is the full 100-unit scale. That relationship is fixed and is a property of the syringe, not of anything you put in it.
For example
1 mL = 100 units on a U-100 syringe. 0.25 mL = 25 units. 0.5 mL = 50 units.
What it is not
A millilitre is not a dose, and it is not a milligram. Two people drawing 0.5 mL can be drawing very different amounts of compound if their solutions were made at different strengths.
Where this comes up: reconstitution, syringe-units