A text style characterized by a horizontal line through the center of all letters. Often used in legal documents to illustrate deletia, it is available to HTML users as the <s> tag. For example, the source
I read Everything2 for the <s>trashy sex</s> dry wit
would display as
I read Everything2 for the trashy sex dry wit
As you can see, strikethrough is useful when you want to pretend-delete something, like what some people do with ^H and ^W in textual message board postings.
Strikethrough is often used by Pokey the Penguin in conjunction with bold, underline and italic (the only other three font styles in Paintbrush not counting subscript or superscript) for extreme emphasis. For example, YES!!!! instead of merely "Yes!" is used.
Of course, even the most primitive versions of the Mac OS could one-up such styles. With the addition of two more styles, TextEdit could spit out bold italic underline strikethrough outline shadow text. Unfortunately, those last two haven't quite made it into the mainstream just yet.