When is it appropriate to abbreviate words? The answer to this question, as with many matters in writing, is not a simple one: It depends on type of content and the degree of the content’s formality.
In technical publications and scholarly journals, abbreviation of statistical information or references to dimensions and durations abounds. Furthermore, the American Medical Association’s manual of style dictates that periods be omitted in many abbreviations. However, in general content intended for professional publication, consider whether to abbreviate, especially in contexts in which multiple various abbreviations might be distracting.
Social titles such as Mr., Mrs., and Ms., are usually superfluous altogether but are abbreviated when they appear, except in generic usage such as “Hey, mister!” Doctor is abbreviated before a name but otherwise spelled out, as is saint. (Note, however, that cities and other geographic designations differ in using Saint or St.; consult a resource to verify the correct style for a particular location.) Military and quasi-military ranks are spelled out or abbreviated depending on context, but as with other titles, they should be spelled out in isolation (for example, “The captain returned the salute”).
Regarding i.e., that abbreviation and its close cousins e.g. and etc. are convenient, but they are no improvement on the English equivalents (“that is,” “for example,” and “and so on,” respectively). Style for scholarly journals is to use the abbreviations in parentheses and spell out the English phrases outside parentheses, but this distinction is not recommended for general-interest publications; avoid them altogether.
Names of countries and other geopolitical entities are usually spelled out as nouns but abbreviated as adjectives (“the United States,” but “the US economy”); note in the previous example that periods in such designations, as in most other capitalized abbreviations, are unnecessary. Designations of thoroughfares, like many other words, can be abbreviated in lists or in graphics where space is at a premium, but generally spell out such terms as avenue, road, and street even when they are part of an address.
Titles of senior corporate executives—CEO, COO, and CIO, for instance—are almost always abbreviated in all references, but VP (“vice president”), SVP (“senior vice president”), and the like are generally spelled out in all instances. Corporate terms such as PR (“public relations”) and HR (“human resources”) can be spelled out or abbreviated depending on context; they’re likely to be spelled out in a formal report and probably will be abbreviated in a casual reference in a mass-market book.
Academic degrees are often abbreviated after a person’s name, but it’s better to refer to someone receiving a bachelor’s or master’s degree or a doctorate rather than a BA, an MA, or a PhD. Likewise, “curriculum vitae,” or the plural form “curricula vitae,” is preferable to CV.
References to media such as CDs and DVDs are ubiquitous (or at least were before they began to give way to online access to music and films), and there’s no need to spell those initialisms out. (You’d likely have to look up that DVD originally stood for “digital video disc,” though the second word has since been supplanted by versatile. And who knows, or cares, that URL stands for “uniform resource locator”? The initialism will do in all cases.) However, words for parts of a book or other printed publication should be spelled out (for example, “In my copy, chapter 6 starts on page 47”), and MS or ms, for manuscript, should be used only in informal contexts.
No. is sometimes used as an abbreviation for number in phrases such as “No. 1”; it’s a compromise between spelling the word out and using the number symbol (#).
The abbreviation for versus, vs., is acceptable in informal content, as is OK. (Okay is a common variant, but the initials are more accurate; the most likely derivation of OK is the jocular misspelling “oll korrect,” the only survivor of a short-lived flurry of such locutions coined during the nineteenth century.)
Terms of distance and duration, such as foot and hour, are generally spelled out when accompanying numerals except in technical writing, as are designations such as Celsius and Fahrenheit; the same is true of phrases such as “miles per hour” and “pounds per square inch.” However, the abbreviations am and pm, often capitalized and/or with periods, are always acceptable, though a phrase such as “one o’clock in the morning” is appropriate for a casual reference in fiction or nonfiction.
Abbreviations are much more likely to be employed in ephemeral publications such as newspapers than in more durable materials such as books, though formality varies widely in the latter format; online usage differs as well. Ultimately, the careful writer will consult a style guide appropriate to the type of print or online publication for guidance or at least will consciously consider the visual and cognitive impact of abbreviation.
(See this post for more guidance on abbreviations, and search for “abbreviations” on DailyWritingTips.com for additional assistance.)