When performing legal research you may need to determine whether a term in a contract or legal document has a special meaning -- that is, whether it's a legal term of art. You may also need to verify that a word used in the document, or which you plan to use when drafting or revising a legal document, is the correct choice. So of course, you turn to a legal dictionary for a definition.
Some of the most famous legal dictionaries are available only for payment, but here are some online dictionaries that are reasonably complete and reliable:
Law.com - Search for the definition of a legal term, or search for definitions that include the term.
FindLaw - Search for the definition of a legal term.
Although other dictionaries and glossaries are available online, many are no longer maintained and have become out-of-date, and many others are incomplete.
Black's Law Dictionary
Please be cautious about using any online source that claims to be offering Black's Law Dictionary. The only source for the current version is the publishing house, Thomson Reuters, which sells the dictionary in book form and as an app. Any other source is using a version that has fallen out of copyright, most often the second edition that was published in 1910, more than a century ago. While an antiquated dictionary may still accurately define some legal terms, for others it can be potentially misleading or completely inaccurate.
Although less complete, glossaries can sometimes be a useful source for researching legal terminology. They can be particularly useful if you can't quite recall the term that you're looking for, as you can browse through a list of terms to try to refresh your memory.