Excess volume
Also known as: VE, excess molar volume, objętość nadmiarowa
A term from physical chemistry: the difference between what a mixture actually occupies and what it would occupy if the volumes of its components simply added up.
The sign tells you everything. Negative — the mixture shrinks, as ethanol and water do, and that is contraction. Positive — it takes more room than the sum, because the molecules of the two liquids get in each other's way instead of packing together. Zero means an ideal solution, a limiting case that real pairs of liquids only approach.
One asymmetry is worth remembering: mass is always additive, volume never carries that guarantee. Volume depends on how tightly the molecules can be arranged, and that changes with the composition. Which is why, in distilling, the scale is trusted and the measuring jug needs either a correction or a table.