What I mean by that is this: I've learned about widows and orphans since 1990, but only in a very general way, since the concept doesn't apply to anything I do. But I knew that you weren't supposed to have a single line from the end of a paragraph roll over to appear at the top of the next page. The rest of it was a little vague, and I wasn't sure if that was a "widow" or an "orphan." Probably, I thought, the other term referred to starting a new paragraph with a single line at the bottom of the previous page.
I assumed that, once I started to investigate this seriously, I'd be able to figure out which were the widows and which were the orphans. It turned out to be much more complex than that. I checked several sources, including usually reliable ones like The Chicago Manual of Style and Merriam-Webster's Collegiate Dictionary, and the main thing they agree on is that a single line from the end of a paragraph is a "widow" when it appears at the beginning of the following page. That's about it.
Chicago also says that the first line from a new paragraph should not appear alone at the end of a page, but it's not clear if this is considered a "widow" as well. They define an "orphan" as a single word (or, even worse, a part of a hyphenated word) at the end of a paragraph which appears on its own line (so, it's a question of text wrapping, not of pagination at all).
Other sources consider an orphan to be the reverse of a widow, as mentioned above. Webster's has a typographic definition of "widow" ("a single usu. short last line, as of a paragraph, separated from its related text and appearing at the top of a printed page of column"), but doesn't mention any corresponding definition for "orphan" at all. Given the confusion, I can see why they decided to duck the issue.
Chicago also advises that you can add or remove line or two of text from a page to prevent these sorts of things, but that facing pages should be of the same length (which I agree with), and that you can also fiddle with the line spacing (well, they don't use the word "fiddle") in order to achieve this, which I strongly disagree with. Adjusted line spacing, even slightly squeezed or stretched, looks terrible, and I'm not going to do that. That's like telling a pianist, "Hey, if your pinky won't stretch to the B key, hit the B flat instead, it's almost the same note."
So, tentatively, here's what I think. A single line from the end of a paragraph starting a new page is a worse situation than the reverse, since it will usually be a short line (the last line of a paragraph will seldom reach the right margin, for obvious reasons). The line which begins a paragraph will in any normal circumstance be a full line, for even more obvious reasons. So, that's less of an eyesore. Also, I think a very short and/or partial word should not appear on a line by itself. That's about all I intend to worry about.
By the way, one source said that the mnemonic device you can use to remember which are widows and which are orphans is that orphans have no past (because they are the beginning of the paragraph), and widows have no future (because they are the end of a paragraph).
I'll say no more about that, except to say that it reminds me of the line from The Importance of Being Earnest when Algernon, speaking of a widow, says, "I hear her hair has turned quite gold from grief."