A unit formerly used by chemists to express the concentration of a solution. A 1-demal solution contains 1 gram-equivalent of solute in 1 cubic decimeter of solvent at 0° C. Symbol D. The unit was proposed in 1924 by Parker and Parker.¹

Note that the only difference between expressing concentration in demals and as normality, a much more common measure, is that for normality the volume of solvent is measured in liters. The sole reason for the existence of the demal was the uncertainty introduced by the divergence in the sizes of the liter and the cubic deciimeter. People assumed the liter was a cubic decimeter, but after 1901 it was not. In 1901 the Third CGPM declared

1. The unit of volume, for high accuracy determinations, is the volume occupied by a mass of 1 kilogram of pure water, at its maximum density and at standard atmospheric pressure; this volume is called “liter.”

Comptes rendus des séances de la Conférence Générale des Poids et Mesures, page 38.

Thus the liter was actually based on the International Prototype of the Kilogram. The cubic decimeter, of course, was not. As measurements became more precise it became clear that the liter, as so defined, was bigger than a cubic decimeter, actually, 1.000 028 cubic decimeters. In 1964 Resolution 6 of the 12th CGPM reestablished “liter” as simply a special name for the cubic decimeter, and the need for the demal vanished.

Judging by Google's n-gram viewer, the demal was mainly used between 1930 and 1950.

1. Henry C. Parker and Elizabeth W. Parker.
The calibration of cells for conductance measurements.
III. Absolute measurements on the specific conductance of certain potassium chloride solutions.

Journal of the American Chemical Society, vol 46, no. 2, pages 312-335 (February 1924).
doi: 10.1021/ja01667a007.