Me, I think it’s when you accidently strike the wrong key, usually an adjacent one. I don’t believe mis-spelling counts. Grammar mistakes are not typos. Hyphenating words that shouldn’t be is not a typo. 
Peace,
mangeorge
Typing teh instead of the is a typo.
Typing your instead of you’re is a typo, unless you’re ignorant, in which case it’s a grammatical error.
A typo is when you have too many threads you want to reply to, or have too many ideas pooping into your ehad all at once, and you want to get them on text as quickly as poosible.
…
And yes, upon re-reading, I actually did make those typos… :o
So ending “What’s a typo” with a full stop and not a question mark is a typo.
No, it isn’t. That’s exactly what I’m talking about.
I think we’re on the honor system for typos. If you are typing quickly you might type “phonetically” the way it sounds in your head and make mistakes that go beyond the simple missed keystrokes. If you really know it’s “their” and not “there” but you were in a rush, that’s not a mistake, it’s a typo. You wouldn’t have made the same mistake longhand, it was a mistake that was the product of how fast & unconsciously you type. But from the outside, it just looks like you’re dumb. However, people who know and love you will believe you when you say “oops typo.”
Oh, so I disagree. Grammar mistakes aren’t typos in my book, but substituting a homonym, or typing “ov” to mean “of” would be a typo in my rulebook.
//hijack etymology of typo? hijack//
Short for “typographical error” – and error made when setting something into type.
If the author wrote “the” and the typographer wrote “teh,” it’s a typo. If the author wrote “teh” and the typographer wrote it as “the,” that’s a typo, too, and the typographer would be reprimanded. Correcting the text is the copy editor’s job – the typographer is supposed to follow the text without variation.
FWIW, I agree with your OP. To me, a typo is something that results only from the accidental striking of the wrong key. “Their” instead of “there” doesn’t qualify.
There should be a name for that homonym confusion thing, though, because sometimes it’s not so much a grammatical error as it is a temporary hand-eye coordination thing. My brain says “their” and my stupid fingers type “there.” It annoys me horribly when someone writes “to” when they mean “too” but knowing that the majority of stuff I read is written by people who actually DO know the difference leads me to believe that there was a brain fart, rather than a lack of knowledge.
One that catches me sometimes isn’t even a homonym - when I type the word S-O-M-E, my fingers sometimes (heh, just did it right there!) type S-O-M-E-O-N-E, because my fingers and brain are out of synch. Not strictly a typographical error as much as a mental typo - we could call 'em mentos, but I think someone has that trademarked 
I think a typo is any kind of mistake you make when typing that you wouldn’t make when writing, or that you’d catch when editing.
Having said that, I am a great, great copy editor, but kind of a lousy typist. Well, not too bad–fast, but prone to putting the wrong word in, at times. Example: I used to be a business reporter and one of my very common mistakes was to write “wholesale and resale” when of course I meant, and would have said, “wholesale and retail.”
But I do a lot of things when typing that I edit out, such as overuse of adverbs, misuse of apostrophes (i.e., “apostrophe’s”–I know that is completely wrong but I do it when typing. My fingers just throw it in).
The odd thing is that, having typed them, I have a hard time seeing my mistakes. That is why, at the paper where I was a copy editor, everybody else’s stories had a 100% chance of having no typos, no embarrassing things like “a whopping $10 in sales tax” when they meant to say “a whopping $10 million in sales tax,” but I had to rely on the typesetters to catch mistakes in my copy. Once it was set in type it looked different enough that I could usually see the errors, both typos and thinkos (a “thinko” being something like, “she likes and enjoys music”).
I am now going to (probably) hit “submit reply” instead of “preview post,” not that preview helps. I only see the typos when the thing is posted.
Anything I have to correct before we go to press is a typo. Defining it (while fun!) doesn’t make the shit better when it hits the printed fan.
And don’t get me started on discretionary hyphens.
Typing “banhattan” for “manhattan.” Funniest SDMB typo ever.
Yeah see? So where does something like “penis ensues” fit in? Not really a typo, but it’s definitely not what the poster meant to type. Freudian slip, perhaps, but more likely an automated finger response.
I like “thinko.”
I like “thinko” too. Let’s start using it.
As long as we’re on the subject, one word I refuse to accept is “noone”. I don’t care how commonly it’s used. I asked about the usage of “noone” awhile back on the SDMB and several posters claimed that it wasn’t a typo but that it was a real word.
I hope not.
Lapsus dactylographi?
You watch your language, young man. I’m a respectable old broad.
So what do we have, so far?
Transposition errors will likely stir little argument as being legitimate typographical errors, and I think misspellings by virtue of hitting the wrong, perhaps adjacent, key probably draw much the same judgement. Inappropriate punctuation might make the Dopers’ cut*.
The brain farts, though, seem to be the grey area. I know the difference between “your” and “you’re” and the homonyms of “there,” and I’ve certainly fired off a missive or two to these boards (on preview I see that I initially typed in the singular “board” with the plural “these” [Arghh! and then typed “withe” for “with the”] with the wrong form included.
Inserting an entirely out-of-the-blue word, as in “penis ensues,” definitely falls in the brain fart category, but might be phenomenon in need of another name.
Just to confuse the issue a little more, I’ll throw in that I once worked with a woman who was an exceptionally fast typist. If you walked into her office while she was whippin’ out a contract and said, “Hi, Barb!” she’d type that as you spoke it. She couldn’t help it (and boy did it piss her off!). Could that be called a typo?
*[sub]Y’all wanna debate apostrophes with plurals, again?[/sub]
And, as long as we’re at it, how about those disjointed sentences and/or paragraphs that only became possible once word processing appeared?