Definition
Justification
Justified text is flush on both the left and right margins, achieved by varying word spacing. Most printed novels are justified; most ebooks are better left ragged right.
Justification is the print convention because a straight right edge reads as finished and professional on a fixed page. It costs something: the renderer must stretch or squeeze word spaces line by line, and without hyphenation those adjustments become visible as loose lines and rivers. In EPUB the page width is unknown and reflows on every device, so forcing justification without hyphenation support is how ebooks end up with alarming gaps mid-sentence.
Related tools & guides
See it handled in your own manuscript
Ceeryo sets your .docx as a book — justified and hyphenated text with widows, orphans and rivers controlled, drop caps, running heads — and shows you every spread before you pay anything.