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The degree of ionization refers to the proportion of neutral particles such as those in a gas or aqueous solution, that are ionized into charged particles. A low degree of ionization is some times called partially ionized, and a very high degree of ionization as fully ionized.
Ionization refers to the process whereby an atom or molecule looses an electron, resulting in two oppositely charged particles, (1) a negatively charged electron and (2) a positively charged ion.
Chemistry usage
The degree of ionization, or α, is a way of representing the strength of an acid, often represented by the Greek letter alpha. It is defined as the ratio between the number of ionized molecules and the number of molecules dissolved in water. It can be represented as a decimal number or as a percentage. One can classify strong acids as having ionization degrees above 50%, weak acids with α below 5%, and the remaining as moderate acids, at a specified molar concentration.
Physics usage
In gases and plasma, the degree of ionization refers to the proportion of neutral particles that are ionized into charged particles. For example, when electricity passes through a novelty plasma ball, perhaps 1% of the gases are ionized (sometimes referred to as partially ionized). Our Sun and all stars contain largely hydrogen and helium gases, that are fully ionized into electrons, protons (H+) and helium ions (He++).
A gas may begin to behave like a plasma when the degree of ionization is as little as 1%.
History
Ionized matter was first identified in a discharge tube (or Crookes tube), and so described by Sir William Crookes in 1879 (he called it "radiant matter")Crookes presented a lecture to the British Association for the Advancement of Science, in Sheffield, on Friday, 22nd August 1879 Crookes tube "cathode ray" matter was subsequently identifed by English physicist Sir J.J. Thomson in 1897Announced in his evening lecture to the Royal Institution on Friday, 30th April 1897, and published in Philosophical Magazine, 44, 293 *," target="_blank" >and dubbed "plasma" by Irving Langmuir in 1928 I. Langmuir, "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1929PhRv...33..195T&db_key=PHY&data_type=HTML&format=&high=42ca922c9c29098 Oscillations in ionized gases," Proc. Nat. Acad. Sci. U.S., vol. 14, p. 628, 1928, perhaps because it reminded him of a blood plasma G. L. Rogoff, Ed., IEEE Transactions on Plasma Science, vol. 19, p. 989, Dec. 1991. See extract at http://www.plasmacoalition.org/what.htm.
Footnotes
"Degree of ionization"