There are a number of legal dictionaries online:
Duhaime's Law Dictionary:
http://www.duhaime.org/LegalDictionary.aspx
Find Law:
http://dictionary.lp.findlaw.com
Everybody's Legal Dictionary:
http://www.nolo.com/lawcenter/dictionary/wordindex.cfm
Legal Lexicon's Lyceum:
http://www.lectlaw.com/def.htm