U-100 syringe
A U-100 syringe is one whose scale is divided so that 100 marks equal exactly one millilitre of liquid.
The 'U' stands for units, and the 100 describes the scale: 100 units per millilitre. Each unit mark is therefore one hundredth of a millilitre. The scale was designed for U-100 insulin, where that ratio conveniently matches the insulin concentration — but the syringe itself measures only volume.
This is why a unit figure quoted by someone else is meaningless on its own. It describes a volume drawn from their solution, at their concentration, not an amount of anything.
For example
A 0.3 mL barrel U-100 syringe carries 30 unit marks. A 1 mL barrel carries 100. Both measure the same thing at the same scale; they differ in how much they hold and how finely the marks are spaced.
What it is not
A unit is not a dose and not a measure of drug. It is a volume mark. Transferring a unit number to a differently scaled syringe, such as a U-40 device, produces a different volume entirely.
Where this comes up: reconstitution, syringe-units, reverse