Using italics and quotation marks to emphasize words and phrases is a useful technique, but for the sake of clarity, such formatting should be used only in the manner described in this post.
Italics help readers understand that a word is being presented as the label for a concept and not as a concept itself, just as when one italicizes a letter when one writes “the letter a” or “It looks like a z to me.” For example, note the difference in the use of the key word in these sentences:
Monarchy is a form of government headed by a king or queen.
Monarchy is defined as “a form of government headed by a king or queen.”
The first sentence begins by using a word to identify a form of government—a concept. The second sentence defines the word; it is used to refer not to the concept of the form of government but to the lexical label for the form of government: “One form of government is monarchy,” but “The word is monarchy.”
Note that the second sentence does not need to specify the word-as-a-word status of monarchy (“The word monarchy is defined as . . . .”), just as I don’t need to do so in the sentence you are reading right now, but sometimes, as in the last sentence in the previous paragraph, a phrase such as “the word” occurs naturally. Here’s another example in which a word’s status as a word is explicit: “I think that the word you are looking for is irony.” And here are two examples that point out the distinction between a word describing a concept and a word used as a word: “Such a word is called a misnomer,” but “The word for this is misnomer.”
What if the term consists of more than one word? The editor’s desire for order and consistency supports italicization, but enclose phrases as phrases in quotation marks. (Says the grammar cop, “I don’t make up the rules, ma’am—I just enforce them.”) For example, one would write, “The phrase in question is ‘plausible denial.’” (Use double quotation marks for a phrase as a phrase such as “plausible denial”; I used single quotation marks in the example because they appear within a quotation that uses double quotation marks.) Note the distinction between phrase as label for concept and phrase as phrase in these examples: “Such office settings came to be called cube farms,” but “Someone came up with the phrase ‘cube farm.’”
In addition, in textbooks and instructional manuals, words and phrases are often italicized when the concepts they represent are introduced, even if they are not being identified as words as words. (Sometimes, they are formatted in boldface, but usually this emphasis indicates that these terms are introduced as new vocabulary and defined in a glossary.) In such cases, the emphasis is provided in the first reference only; all subsequent uses of the term are not italicized.
Some examples in which words might be emphasized in an educational context (but ordinarily need no emphasis) follow:
“In general, this concept is called contradiction or paradox.”
“This is an example of an idiom.”
“Distinguish between the concepts of fitness and adaptation in evolution.”
Two key exceptions in the use of italics for emphasis are writing proper nouns (for example, “The second p in PayPal is also capitalized”)—though italicizing proper nouns may enhance clarity—and conveying speech (“Many people say ‘myself’ when they should say ‘me’”); communicating what one might write is more of a gray area, but in these posts, I italicize in such cases (“Insert that into the sentence”).
Italics are used for other forms of emphasis: For example, foreign words (and phrases) are italicized to emphasize their outsider status. (However, many such terms have been adopted into English, so check a dictionary before formatting a foreign-seeming word or phrase; if it has an entry, it is considered an English term and should not be emphasized.) Again, as in the case of introduced concepts and vocabulary, italicize such terms on first reference only (unless just a few instances are scattered throughout a long piece of content; use your judgment in such cases).
Italics are also employed to signal an emphasis that would not otherwise be communicated. For example, in the sentence “It was him!” the default emphasis is on him, conveying that the focus of the sentence is on the identity of a person. But “It was him!” shifts the emphasis to the verb, communicating that one’s earlier suspicion has been confirmed.