Black's Law Dictionary (9th ed. 2009) is the standard US legal dictionary. Because it is not freely available online, you might find one of the following free legal dictionaries useful.
The legal community has its own citation manual: The Bluebook: A Uniform System of Citation (20th ed. 2015).
Introduction to Basic Legal Citation provides basic guidance in creating a legal citation.
Other academic disciplines use different citation systems, including the APA, the Chicago Manual of Style, Harvard, and MLA. Check with your instructor for information on which citation system s/he prefers. The UW Libraries provides guides to several nonlaw citation systems.
Contact the Gallagher Law Library reference librarians for help deciphering legal citations.