When an output line is nearly full, it is uncommon for the next word
collected from the input to exactly fill it—typically, there is room
left over only for part of the next word. Hyphenation is the
process of splitting a word so that it appears partially on one line,
followed by a hyphen to indicate to the reader that the word has been
broken, and its remainder on the next. Hyphenation break points can be
manually specified; GNU troff
also uses a hyphenation algorithm
and language-specific pattern files (based on TeX’s) to decide which
words can be hyphenated and where.
Hyphenation does not always occur even when the hyphenation rules for a word allow it; it can be disabled, and when not disabled there are several parameters that can prevent it in certain circumstances. See Manipulating Hyphenation.