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. The process of splitting a
word so that it appears partially on one line (with a hyphen to indicate
to the reader that the word has been broken) with its remainder on the
next is hyphenation. Hyphenation points can be manually
specified; GNU troff also uses a hyphenation algorithm and
language-specific pattern files (based on those used in TeX) 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.