It's very subjective. Merriam Webster defines it as a modulation of the voice expressing a particular feeling or mood.
Perhaps best defined when a parental unit inquires, "You're going out in that?" when on your way to an evening social function.
See, they're not asking you if that is indeed what you're wearing, because they can see these are the clothes (or lack thereof, really) you have chosen to wear.
They're asking, "You're (my child, my flesh and blood, I carried you for nine months) going out in THAAAAAAAT (It's see-through, for the love of Christ, how dare you, you harlot)?"
That's tone. For most people, it's easy to gauge.
But what about online?
A recent study published in the Journal of Personality and Social Psychology finds that we only have a 50/50 chance of correctly interpreting the intended tone of an e-mail, because we infer the tone of messages we receive based on our own mood, stereotypes and expectations.
I am notoriously bad at inferring tone over text, because I constantly assume I'm wrong, I've angered someone, and people hate me. However, it seems most of my friends interpret my own online communications as hostile, sarcastic and unhappy.
And this is how flamewars get started.
Flamewars, my friends, are the direct result of massive misunderstandings online. Or, they are completely intentional. Either way, it all comes straight back to tone.
For example, an old friend of mine has a girlfriend I hate. This friend posted to his blog a "Happy Birthday" message to his sister. The girlfriend said in a comment to his post something along the lines of, "How come I never got a Happy Birthday post?"
She may have meant that completely innocently. It may have been a simple inquiry, or a slight jab at her boyfriend for not treating her birthday as importantly as he treated his sister's.
Or she could be an idiot who has no respect for the bond between siblings, doesn't understand that she will never be as important to him as his sister, and shamelessly insults and manipulates her boyfriend in front of the Internet At Large.
Which is how his sister and I took it. (There's that tone thing again.)
His sister, who happens to be my best friend, came out claws first and ripped the girl a new one. Then she called me to tell me the URL where it was going down, and I joined in. The girlfriend comments back with something snippy, I add my two cents with a generous helping of sarcasm, my friend adds a twist of hostility, stir, shake, pour.
That is the recipe for a flamewar.
To make matters worse, she made a post to HER blog, which my friend and I monitor, about "her boyfriend's losers, oops, I mean friends" so of course we took our fight there.
The ensuing argument included phrases like "u don't know me" "If you post it to the Internet, hon, its up for grabs" "self-important tirade" and "my dad is dead and my mom is a battleaxe."
This ended very, very badly as you might imagine. And had nothing at all to do with the initial Happy Birthday post.
What gets people around this, though, in an increasingly text-heavy era, are emoticons-combinations of punctuation marks used to signify emotions.
>'o'< (That's a cat.)
See how annoying those are? But, for the most part (except in cases of sarcasm), they clear up tone in the absence of an actual, you know, human face.
Hey, it's the best we can do until we get, like, virtual reality suits capable of 3D interaction. And holograms.