Definition
Widows and orphans
A widow is a paragraph's last line stranded at the top of a page; an orphan is a first line left alone at the bottom. Professional typesetting prevents both by adjusting page breaks.
Single stranded lines break reading rhythm and look careless, so composition engines enforce minimums (commonly two lines together on each side of a page break). This is one of the invisible-until-absent qualities separating typeset books from exported documents.
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.