S: (n) chemistry (the chemical composition and properties of a substance or object)
S: (n) material, stuff (the tangible substance that goes into the makeup of a physical object)
S: (n) phlogiston (a hypothetical substance once believed to be present in all combustible materials and to be released during burning)
S: (n) mixture ((chemistry) a substance consisting of two or more substances mixed together (not in fixed proportions and not with chemical bonding))
S: (n) atom ((physics and chemistry) the smallest component of an element having the chemical properties of the element)
S: (n) chemical element, element (any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter)
S: (n) activator ((biology) any agency bringing about activation; a molecule that increases the activity of an enzyme or a protein that increases the production of a gene product in DNA transcription)
S: (n) substrate (the substance that is acted upon by an enzyme or ferment)
S: (n) element (one of four substances thought in ancient and medieval cosmology to constitute the physical universe)
S: (n) medium (an intervening substance through which something is achieved)
S: (n) medium ((biology) a substance in which specimens are preserved or displayed)
S: (n) fluid (a substance that is fluid at room temperature and pressure)
S: (n) volatile (a volatile substance; a substance that changes readily from solid or liquid to a vapor)
S: (n) essence (any substance possessing to a high degree the predominant properties of a plant or drug or other natural product from which it is extracted)
S: (n) medium (an intervening substance through which something is achieved)
S: (n) medium (a state that is intermediate between extremes; a middle position)