7 Italic, roman, and other type treatments
7.4 Bold type
Bold or boldface is a thick typeface like this. Bold type is indicated on hard copy by a wavy underline. Where a distinction is to be made between two bold typefaces (e.g. bold and semibold), the convention is to use a double wavy line for bold and a single one for semibold.
Bold may be used instead of italic to highlight a newly introduced term, often one that is going to be defined or explained:
The Pharisees were sincere and pious Jews … The Sadducees were a group of aristocratic Jews … The Essenes had serious disagreements with both the Pharisees and the Sadducees.
This device is used particularly in textbooks, guidebooks, and other educational or instructional texts, less so in more literary contexts:
Bold is often used for headwords in dictionaries and encyclopedias; for certain components of citations in bibliographies and reference lists; in indexes to draw attention to types of reference or important references; to indicate cross-referencing generally; for vectors and matrix symbols in science and mathematics; and in titles and headings.
Avoid using bold for emphasis in the course of normal printed matter, as the effect is usually too startling in running text; avoid typographical distinction altogether, or prefer instead the less obtrusive italic.