http://www.unc.edu/~rowlett/units/index.html
A web site giving definitions of scientific units, conversions and other information. No registration required.

https://medlineplus.gov/mplusdictionary.html
A freely available medical dictionary from the US National Institutes of Health. Links to other useful web sites.

http://en.wikipedia.org/
A free on-line encyclopedia, patchy and erratic in coverage but contains some useful material on a very wide range of topics.