* Dichotomy: A division into two parts or categories.
* Dichromatic: Having two colors.
* Didactyl: Having two fingers or toes.
* Didymus: A twin.
* Diptych: A two-part object, often a painting or sculpture.
* Diphthong: A combination of two vowel sounds.
* Disyllable: A word with two syllables.
* Digenesis: The formation of two distinct groups of organisms.
* Dimer: A molecule composed of two identical subunits.
* Dioxide: A compound containing two oxygen atoms.
* Diphyletic: Originating from two different ancestral lines.
These are just a few examples, and the prefix "di-" can be used in many other words to indicate a doubling or a division into two.