- In chemistry, a theory advanced by Berzelius that every compound, no matter how many elements enter into it, is composed of two parts, one electrically negative, the other positive; still applicable, with modification, to polar compounds, but inapplicable to nonpolar compounds.
- In hematology, the concept that blood cells have two origins, lymphogenous and myelogenous.
- The theory that the mind and body are two distinct systems, independent and different in nature.
[L. dualis, relating to two, fr. duo, two]