In typesetting, widow refers to the final line of a paragraph that falls at the top of the following page [or column] of text, separated from the remainder of the paragraph on the previous page [or column]. Typesetting involves the presentation of textual material in graphic form on Paper or some other medium. The term can also be used to refer simply to an uncomfortably short (e. g. , a single word or two very short words) final line of a paragraph. 
A related term, orphan, refers to the first line of a paragraph appearing on its own at the bottom of a page [or column] with the remaining portion of the paragraph appearing on the following page [or column]; in other words the first line of the paragraph has been "left behind" by the remaining portion of text.
Note that a widow, by the second definition above, can also fall at the bottom of a page, in the sense that the page ends on a very short line at the end of a paragraph.
One easy way to remember the difference between an orphan and a widow is to remember that orphans "have a future but no past," while widows "have a past but no future" just as an orphan or widow in life. 
Writing guides generally suggest that a manuscript should have no widows and orphans even when avoiding them results in additional space at the bottom of a page or column. Some techniques for eliminating widows include:
An orphan is cured more easily, by inserting a blank line or forcing a page break to push the orphan line onto the next page to be with the rest of its paragraph. Such a cure may have to be undone if editing the text repositions the automatic page/column break.
Most full-featured word processors and page layout applications include a paragraph setting (or option) to automatically prevent widows and orphans. When the option is turned on, a widow is forced to the top of the next page or column; and the line preceding an orphan is forced to the next page or column with the last line. This automatic adjustment to a page's layout can be a source of frustration for someone who is unaware of why text is shifted from one page to the next.