When an offensive, harsh or blunt expression is replaced with an mild, indirect or vague one.
Visual euphemisms exist, too, such as the one used for Akiyama Mio's accidental pantyshot in the K-On! anime (a stripped bowl of rice) or when swastika or the hammer and sickle are replaced with more innocuous symbols to avoid offending the audience and/or so the work won't run afoul of laws banning the public display of either symbols.
An orthographic euphemism is spelling a word deliberately wrong (e.g. "porn" becomes "pr0n", "n0rp" or "prawn") to circumvent censorship algorithms on the usenet, websites, social media apps, etc.
