A named quantity in terms of which other quantities are measured or specified. Quantities for which units are commonly defined include Area, Length, Temperature, Time, Volume, etc. Examples of units in each of these categories include the Acre, Meter, Kelvin, Second, Eric Weisstein's World of Astronomy Liter, etc. The references below give several handy www pages which perform conversion between different units of the same type.
Avoirdupois System of Units, cgs, Imperial System, MKS, SI, Troy System of Units, United States System of Liquid and Dry Measures
References
--. "Unit Conversion." http://www.soton.ac.uk/~scp93ch/refer/convform.html.
Cardarelli, F. Scientific Unit Conversion: A Practical Guide to Metrication. 1997.
Dresner, S. Units of Measurement: An Encyclopedic Dictionary of Units, Both Scientific and Popular, and the Quantities They Measure. 1972.
Entisoft. "Entisoft Units Measurement Conversion Calculator (The Internet Version)." http://www.entisoft.com/unitscgi.htm.
James and James. "Denominate Numbers." In Mathematics Dictionary, 3rd ed., Multilingual Edition. Princeton, NJ: Van Nostrand, pp. 427-428, 1968.
Jerrard, H. G. and McNeill, D. B. Dictionary of Scientific Units: Including Dimensionless Numbers and Scales. 1992.
Kirste, B. "Conversion of Units/Umrechnung von Einheiten." http://www.chemie.fu-berlin.de/chemistry/general/units.html.
NIST. "The NIST Reference on Constants, Units, and Uncertainty: International System of Units (SI)." http://physics.nist.gov/cuu/Units/.
Rowlett, R. "How Many?: A Dictionary of Units of Measurements." http://www.unc.edu/~rowlett/units/.
Weisstein, E. W. "Books about Units." http://www.ericweisstein.com/encyclopedias/books/Units.html.
