A subword of a word 
, in the sense used for the Bruhat
order, is a word 
obtained by deleting zero or more letters without changing the order of the remaining letters.
Thus 
;
deleting all letters gives the empty word
(Björner and Brenti 2005, p. 33).
For example, ac is a subword of abc, although its letters are not consecutive. This use of subword means a subsequence of the letters. Other contexts use the term for a substring, so the convention must be specified.
For a Coxeter group, the Bruhat order satisfies 
iff a reduced word for 
is a subword of a reduced
word for 
(Björner and Brenti 2005, p. 34).
