The variable blank space inserted between words, whose ideal width depends on the typeface, type size, and measure.
Definition
The word space is the blank interval between words in typeset text. Though invisible, it is one of the most critical elements in typography — its width directly affects readability, texture, and the color of the page. In well-set text, word spaces should be just wide enough to separate words clearly without creating rivers of white space or breaking the line into isolated clusters. Bringhurst recommends a word space roughly equal to the thickness of the letter i in the typeface being used. In justified text, word spaces are variable (stretched or compressed to fill the line), while in ragged-right setting they remain constant.
Source
No access — Robert Bringhurst — p. 347:
The space between words, which varies with the font and is adjusted during justification.