Diatomaceous earth: (also known as kieselguhr) the skeletal remains of diatoms, tiny single celled plankton-like animals.  Composed mostly of silica, microscopically these tiny skeletons are very rough, which gives them an incredibly large surface area.  Diatomaceous earth is therefore extremely absorbent.  It has also come into use lately as an insect killer.  When ingested by small insects (like caterpillar larvae) the sharp, hard skeletons will puncture the digestive tract of the insect, causing it to die.  The microscopic skeletons are far too small to cause any damage to the digestive tract of larger animals, such as mammals or birds.  In fact, diatomaceous earth is often used as a mineral supplement.  More information about diatoms is available here.

Dissociate: to break down.  The term is commonly used for reactions in which a substance forms ions in water.  For example, sodium chloride will dissociate into sodium and chloride ions when it dissolves in water:

NaCl (s) Na⁺ (aq) + Cl^- (aq)

Substances that are acids or bases react with water when they are dissolved to form ions in a process known as the Brönsted-Lowry reaction.  This is often also referred to as dissociation.

H₂CO₃ (aq) + H₂O H₃O⁺ (aq) + HCO₃^- (aq)

In this example carbonic acid (H₂CO₃) would be said to dissociate into bicarbonate (HCO₃^-) ions.

Dimer: a molecule created by chemically joining two identical simpler (monomeric) units.  N₂O₄ is a dimer of two NO₂ units.  Ethylene (C₂H₆) is a dimer of two methyl radicals (CH₃).

Ditnitorgen tetroxide: see nitrogen tetroxide

Dynamic: tending towards change, or in motion, as opposed to static.