Provides not only definitions but also gives a taste of the history & politics behind the units of weights & measures.
Oxford Reference Online provides access to several science and technology resources. They may be searched together at the link above or browsed individually.
From Credo Reference, the Reader's Guide to the History of Science is an encyclopedia covering individuals, institutions, disciplines, and general themes and central concepts. Each entry includes a detailed bibliography to locate addition information. This resource may be searched or browsed.
From Babylon Ltd., Science & Technology Dictionaries provide definitions of science and technology terms from one or more dictionaries. Enter a term and receive definitions from several dictionaries. Subjects covered include agriculture, astronomy, biology, botany, chemistry, computer & internet, earth science, energy, engineering, mathematics & statistics, physics, and zoology.