When trying to learn about and understand law, one of the most important aspects of it is the legal definition of a word. Is there a source in which the courts refer to when looking to define a word? If there is, what is it?
Courts look to primary authority first, and then to secondary authority if ambiguity remains.
Primary Authority providing definition for the legal use of a word would be previous case opinions that give meaning to a word in a given context, how the word is actually defined in the statutes for the state, or, in the case of federal law, the federal statutes or Code of Federal Regulations. Within primary sources, you also consider whether a prior definition is binding on the court (i.e., the court has to follow it) or whether it is merely persuasive authority (that the court can choose to follow, but is not required to follow based on precedent – sometimes call Stare Decisis). Primary authority is binding on a court if the definition comes from a higher court in the direct appellate chain of the deciding court, it is persuasive otherwise.
Secondary Authority is everything else. For example, Black's Law Dictionary, Whigmore on Evidence, or any other legal treatise would also be a secondary source. (All secondary authority is persuasive authority.)
Courts, in absence of either, will look to how a word is commonly used in the context in which it is applied. All seek to give the proper meaning to a word or phrase in light of how it is being used.
In my experience, I have seen such definitions in the preambles to legal agreements and contracts. For example, in a rental agreement, it defines words like dwelling, tenants, landlord, handyman, etc. If the word was defined in the legal document you signed on, then it suppresses the dictionary definition. I highly doubt that dictionaries offer significantly different definitions of the same words. That is where you can find loopholes. If the word was not defined in the contract, and there is an established difference between the local lingo definition and the dictionary definition. So it is up to the lawyers to convince the court/judge/jury to take the “right” definition into account.
Wiki: “In the United States, the law is derived from five sources: constitutional law, statutory law, treaties, administrative regulations, and the common law (which includes case law)”. Therefore, if you read those legal documents, you will notice at the start of each law a definition of each “legal word”. For example, The Massachusetts General Law is divided into Parts, Titles, Chapters, and Sections. Once you start diving into each layer, you will notice that some of the Sections have a “Definitions” section for words and phrases.
As the Wiki entry says, the Black's Law Dictionary is only one of the so called secondary authorities. A Secondary Authority comes to
"explain the meaning or applicability of the actual verbatim texts ofprimary authorities". A
Primary Authority is usually in the form of a document that establishes the law, and if no document exists, is a legal opinion of a court.
From Wiki: The traditional law dictionary with definitions of legal terms serves to help users understand the legal texts they read (a communicative function) or to acquire knowledge about legal matters independent of any text (a cognitive function).
So, the legal definition of words and phrases should be defined as clear as possible in the law itself (e.g., the definition sections of the MGL noted above). If not defined, the Secondary Authority can be used to explain what they supposedly meant. But, I am sure that different resources can have different opinions on what is what. That's where the lawyers come in handy if they can convince the court to accept one meaning over another.