Some publications call lines cut off from the beginning of a paragraph orphans and lines cut off from the end widows. Others reverse the definition and still others call all such lines orphans. The Chicago Manual of Style uses the first definition: a paragraph-ending line that is cut off by a page or column break so that it ends up in the next column is a widow, while an orphan line is one which is cut off from the beginning of a paragraph. "Orphan" can also refer to a word, part of a word or a very short line of text coming at the end of a paragraph.
Most writing guides recommend that a document should not contain orphans or widows, as these break up the flow of the document and can interfere with comprehension. It is generally agreed that empty space is preferable. Recent guides such as the 16th edition of the Chicago Manual of Style allow exceptions if the line is full measure -- that is, if it extends across the whole column or page with no indentation.
Page or column breaks may be added to push the widow onto the same page as the rest of its paragraph. This can leave space at the end of a page or column; space is preferable to orphans and widows but too much space may not be desirable. Other options include rewording the text so that the paragraph is shorter or longer; adjusting page margins; adjusting the spaces between words; adjusting the way that words are hyphenated; changing the page margins; scaling the page slightly (making everything on it bigger or smaller); adding extra material, such as a pull quote or a picture; or adjusting the page margins.
It is usually easier to fix an orphan than a widow. A page break or a blank line can be inserted before the orphan, pushing it onto the next page or column so that it is back at the top of its paragraph. If the text is re-edited later on, however, these fixes may need to be removed. Many page layout applications have a feature that automatically fixes widows and orphans, as do many word-processing packages.