Simply put, marking online text to indicate sarcasm.
Even verbally, sarcasm doesn't always carry over well. Some people just can't recognize it. Some people just can't express it. On the Internet it's worse. While it's bad to act out an emotion by merely stating that you feel that way, on the Internet it is sometimes necessary to indicate emotions such as sarcasm. Here is one way to mark it up:
sarcasm mode on You mean you can't hear voice inflections in text? sarcasm mode off
This is also seen in Faux HTML Tags: <sarcasm> </sarcasm> form. Usually with this kind of sarcasm tag, only the closing tag is used. It's not as much fun, and might even be insulting, to announce in advance that you're being sarcastic, and oh, how the world weeps when you're insulted, right? </sarcasm>
Some people display sarcasm by emphasizing certain words as one would in speech, usually by using italic font. Others however, just use words that are not common in natural speech, such as "Gosh, really?", "My goodness, I never would have guessed that", and "Gee willikers, that's so insightful." Other people have even proposed a sarcastic font.
Roger Ebert has a rather comprehensive essay on this very subject, titled "This is the Dawning of the Age of Credulity", written after people completely failed to appreciate the sarcasm behind an 'interview' he gave regarding Creationism. This, however, was due to Poe's Law.
Users both here and on TV Tropes have been known to mark sarcasm in what they write by potholing the sarcasm in question to this page. Do NOT do this. It's a much better idea to just write in a way that makes the sarcasm clear. Limit the potholes to when it's actually necessary, like for showing the sarcasm in a quote whose sarcasm isn't apparent without the pothole.
Compare and contrast Sincerity Mode.
- Ironically, this essay is against sarcasm, and it gets this through by being entirely sarcastic.