In typography, a widow is a single word as the last line of a paragraph.
The diligent typesetter, desktop publisher or production artist corrects this by adjusting kerning or wordspacing within the paragraph to either pull the widow up a line, or move other words down to the last line. It is tacky to accommodate widows with soft line breaks, which may introduce spacing problems later if the text is edited.
Widows are considered unaesthetic and therefore undesirable. Because of these negative connotations attached to a sex-specific term, I prefer the sex-neutral "orphan". In typography orphan refers to the last line of a paragraph beginning a new page. Although this is a distinct meaning, functionally "widows" and "orphans" present the same problem to the typesetter: a stranded piece of text, isolated from the rest of its paragraph. I find no need for this degree of specificity, though traditional typesetters may inform me otherwise.