What does the term justify mean in Microsoft Word?

When text is "justified" it is arranged so that each line begins, or ends, right under the other, creating an even, vertical edge to a block of text. Text is almost always left justified. That means that each line of text begins the same distance in from the left edge. Text can be right justified as well. Then the amount of space between Words, and even between letters, is adjusted to ensure that each line of text also ends the same distance in from the right edge.
The Wordjustify comes from the world of typesetting.

In left-to-right-reading languages, the vertical margin on the left gives a reader's eye the clue as to where to begin reading the next line.


The right margin can be ragged right or right justified.


Ragged right margins are built by the ends of lines terminating with the end of a Word or a hyphen, usually within a few characters of the line-ending above it and below it.


Right justified margins are 'forced' into alignment, so that the right margin, too, is a vertical margin.


Most right-justified margins produced by editors inexperienced in typesetting standards can result in lines of text that are unaesthetically pleasing and are, in fact, hard for the reader's eye to follow. These forced lines of text can contain too few characters or too many characters, because the software apparently makes the decisions about how to honour spaces between Words, punctuation and hyphens.


Finally, right-justified margins, such as those you can find in published books, are seen when the editor chooses a proportional spacing type font -- the 'm' character takes up more space horizontally than the 'i' character -- and spaces the lines of text far enough apart from each other -- spacing or leading -- so that the reader's eye can 'find' the next line.


The Word justify on a software program menu represents the programmer's effort to deliver a vertical right margin making a series of logical decisions, when indeed the human editor is better equipped to make those choices. The Word is a short form of right justify.