dalton - Same as the atomic mass unit. So named in honor of John Dalton who propounded the atomic theory.
decay - Change of one particle into one or more other particles not caused by interaction with anything outside of itself. Typically particles that decay have a specific half-life, which is the period of time in which half of the particles will decay; which particles actually decay is effectively random.
deci- (abbr. d) - Prefix meaning one tenth of something, especially a unit of measure. See metric system.
deka- (abbr. da) - Prefix meaning ten of something, especially a unit of measure. See metric system.
dendrite - A branched extension of a nerve cell that conducts impulses from adjacent cells inward toward the cell body. A single nerve may possess many dendrites. (Synonym: dendron.)
dendron - See dendrite.
density - Quantity per unit volume, especially mass per volume of space.
destructive interference - Wave interference in which the waves wholly or partly cancel each other's amplitude at a location, so that each wave interferes with the ability of the other to effect an object at that location. With quantum wave interference, one wave can prevent another from interacting with another particle at a point, without affecting its ability to react with other particles once the interference is past (in quantum mechanics, "particle" and "wave" are two ways of describing one thing - see wavicle).
deuterium - The isotope of hydrogen that has one neutron in the atomic nucleus. It's atomic mass is approximately twice that of ordinary hydrogen.
Devonian Period - (410 to 360 million years ago) Forests and amphibians appear. See geologic time.
dimension - (1) Degree of freedom; a direction in which the value of a measurement can vary. In cases where dimensions interact with each other, they are always taken perpendicular to each other, so that a change in position in one of them does not cause a change in position along any of the others. For example, length, width, and height are often used as the dimensions of an object when measuring it. Any of the dimensions can be varied without changing the other two. (2) The allowed range of values of one of the coordinates of a coordinate system while all the other coordinates are held constant.
diploid - Having two sets of chromosomes.
dissolve - To become dispersed as molecular or ionic particles throughout another material, called a solvent. The dissolved material is called the solute. Liquids, solids, or gases can dissolve in other liquids or solids. While liquids or solids may become similarly dispersed in gasses, the result is considered to be a mixture, rather than a solution.
DNA - deoxyribonucleic acid; the material out of which chromosomes are made. It contains the genetic information of the biological organism from which it originates and is capable of self replication. Some types of protists use RNA instead of DNA for storing their genetic information.
domain - Biology. A group of related organisms sharing similar cell structures. For example, eukaryotes, with distinct, membrane-enclosed nuclei containing DNA in chromosomes, form one domain, while prokaryotes, without nuclei or organized DNA, is another. The term "domain" was invented when it was realized that single celled organisms differed far more among themselves than did members of the so-called kingdoms of plants and animals. Little known single-celled species called archaea are in one or more additional domains. See taxon.
Doppler shift - The measured change in the wavelength (or frequency) of a wave due to motion of receiver relative to that of the source of the wave. Can be used to measure the speed of approach or retreat of an object if a wave of known original wavelength can be detected from the object.
dorsal - Of, in, or near the surface of the body of an organism that is to the rear when it's standing erect, or on top when it's lying prone or has all limbs under it.
dyne - The unbalanced force which, acting on one gram of mass, gives it an acceleration of 1cm/sec2. See also CGS system.