Defining a term so the reader shares its meaning
A word the reader may not know, or may read differently from the writer, needs a definition, and the right definition depends on its kind, on the project it serves and on the tests a good definition passes.
Terms that need a definition
Words convey meaning only if the audience understands what the writer intends them to mean. A word that means nothing to the reader baffles them, and a word that seems to mean something other than what the writer meant misleads them. Medical communicators must often discuss concepts unfamiliar to most people, so they must use words that are new to some of their audience, and sometimes familiar words that take on an unusual meaning in a particular context. A good writer introduces such a word properly by defining it at first mention. Lazy writers often fail to give these definitions, and bad writers give bad definitions.
Kinds of definition
There is more than one kind of definition, and each kind serves a particular purpose. A lexical definition is the dictionary's account of what people generally mean by a word, and it is the kind writers and copyeditors meet most often. Where a technical term would be unfamiliar to the reader, the writer should consider giving its dictionary definition to save the reader looking it up. A reportive definition gives the meaning a word carries for a particular group of language users where that meaning differs from the definition in a standard dictionary, as a word may mean something different to doctors than to the public. Where common usage departs from the dictionary definition, the writer may need to give the reportive definition and explain how it differs.
A stipulative definition gives a term a specific meaning within a particular discussion or argument. Some stipulations are arbitrary, as when a scientist coins a word for a new concept, such as Murray Gell-Mann's quark for a new type of subatomic particle, and writers often give an arbitrary meaning to a familiar word. Taken to a ridiculous extreme, the practice produces Humpty-Dumpty words, whose stipulated sense seems deliberately misleading. Other stipulations are restrictive, as when medical writers specify that an elderly person is someone at least 65 years old. Many legal definitions are a restrictive kind called precising definitions, which extend a lexical definition by adding criteria that narrow down the members of the set it describes. A precising definition clarifies precisely what a term denotes, and such definitions are common in medicine, as when a drug label specifies that the dosage recommendations for children should be followed for those 12 years of age or younger.
Many definitions, precising ones among them, clarify whether someone or something belongs in the set a term describes, and definitions relate to sets in several ways. An intensive definition specifies the rules for including members and excluding nonmembers, and an extensive one lists every member. The lexical definition of a starting lineup, for example, is the set of players who will actively participate in a game when it begins. Its intensive definition is a set of criteria, such as whether a person is present at the time and place of the game and is eligible to take part, not listed in injured reserve. The extensive definition of a particular starting lineup is the list of all the players who make it up.
Medical communicators must often deal with theoretical definitions, which describe an abstract concept such as intelligence, and operational ones, which rest on one or more tests of an observable trait, such as an IQ of 120 or higher. Both pose serious philosophical problems. Intelligence has something to do with the ability to solve problems, yet it is questionable whether learning to solve a particular kind of problem makes a person more intelligent, or whether someone who scored 121 on a given test on a particular day is really superior to someone who scored 119. Theoretical definitions resemble Socratic ones, which answer a question of the form what is F-ness?, such as what piety, justice or virtue is. A good theoretical or Socratic definition can be surprisingly hard to give: what health is, or mental health, are important questions even if they are not always answerable. Definitions are sometimes used for poetic or rhetorical purposes. A metaphorical definition is used for artistic effect rather than literal meaning (happiness is a warm puppy), and a loaded definition expresses a value judgement rather than conventional meaning (an embryo is a preborn child).
Ways to explain a term to a lay reader
One strategy that helps a writer reduce jargon for a lay reader is to use in-text definitions and explainers, as in the area in your brain where, the section in your brain responsible for and a disease in which. Another is to use straightforward analogies, similes and metaphors: hoses and pipes for blood vessels, a four-cylinder car engine for the heart's function, servicing a car regularly for preventative health check-ups, and dam walls for resilience. Substitution uses simpler words where possible: kidney for renal, heart for cardiac, bones for skeletal. The writer can also share the content with the target audience, asking non-scientific people to review it. In web copy, the writer can link out to a definition or clarification.
Explained in plain English, aphthous stomatitis becomes aphthous ulcers, painful yet harmless mouth sores that usually go away in a couple of weeks; choledocholithiasis is one or more gallstones in the common bile duct; costochondritis is redness and swelling, or inflammation, of the cartilage in the rib cage; a benign tumour is not cancerous and does not spread to other parts of the body; and the group in a study that does not receive treatment is called the control group.
Fitting a definition to the project
Explaining a technical term to lay readers requires selecting and focusing on particular layers of its meaning. Among specialists, technical terms are shortcuts that house layers of meaning, both knowledge and know-how. Unpacking all of them is extremely difficult, if not impossible, partly because it would take an overly long rendering and more because it may require an educational journey beyond the scope of the document. So a glossary must be project-specific and focus on the layers of meaning relevant to the communication at hand. In a brochure listing reasons for swollen lymph nodes, cancer may be adequately defined as a disease in which some of the body's cells grow uncontrollably, but it may need a different definition where types of cancer, their symptoms, diagnosis and prognosis are being distinguished.
The same term can need a different lay explanation from one project to the next. Intravenous, intramuscular and subcutaneous injection each target a different layer of the body's tissue. For a general description, a glossary may explain intravenous as injected into a vein, intramuscular as into a muscle and subcutaneous as into the skin. Once intradermal enters a project, the glossary needs modifying, and it may then explain intradermal as injected into the surface layer of the skin and subcutaneous as injected into the fatty layer under the skin. Arrhythmia was simply an irregular heartbeat in one project, but in a project about tachycardias its lay explanation had to cover a group of heart rate disorders in which the heart can beat too fast (tachycardia), too slow (bradycardia) or irregularly. An existing glossary may help, but it should not be relied on without careful scrutiny of the purposes and needs it served. It may be used as a basis to expand on: the injection glossary, for example, may add the use and goals of each route, such as delivering a substance directly into the bloodstream for quick action as against slower, more consistent absorption. An existing glossary may not serve the message or the readers' needs, and it may be easier to compile one from scratch.
Testing a definition
An editor checks whether the author's lexical definitions match the dictionary and queries the author where they do not, though a mismatch sometimes has good reasons, as with a stipulative definition.
Good writers and editors ask whether the lexical definitions reflect a standard dictionary, whether the reportive ones reflect actual usage and whether the stipulative ones are psychologically acceptable. A definition should not be circular, either by repeating the word in its own definition, as in a dog is an animal whose parents are dogs, or across a set, as when a cause is something that produces an effect and an effect is something that results from a cause. It should be neither so broad that it applies to too many things nor so narrow that it excludes too many. Medical communicators should avoid metaphorical definitions, especially in work for an international audience. A metaphorical definition is a form of poetic discourse, not a means of technical communication. Medical communicators should also avoid loaded definitions, especially where the purpose is to convey scientific information rather than to express a political opinion.