An abbreviation is a shortened form of a word or phrase; abbreviations of phrases are often composed of the first letter of each word of the phrase (i.e., acronym). To maximize clarity, use abbreviations sparingly.
Also consider readers’ familiarity with the abbreviation before using it. Although abbreviations can be useful for long, technical terms in scholarly writing, communication is often garbled rather than clarified if an abbreviation is unfamiliar to readers. In general, use an abbreviation if (a) it is conventional and readers are likely to be more familiar with the abbreviation than with the complete form and (b) considerable space can be saved and cumbersome repetition avoided.
Periods are not used with abbreviations in APA Style (e.g., write “FBI” and “PhD” not “F.B.I.” and “Ph.D.”).