Pilcrow

"Pilcrow" is the official name for what I've always just knows as a paragraph mark; that symbol that you find in proofreading and occasionally in some of the more pretentious magazines to indicate that a block of text ought to be broken up into paragraphs.  It looks like this: ¶.  Originally, it was written as a capital "C" with a slash, an abbreviated form of the Latin word for "chapter" (according to Wikipedia).  The single (and double) slash often indicated an instruction from a primary scribe (editor) to a later stage in manuscript production, so a C with two slashes would mean, as it does now, "insert a new paragraph here."  You can still see traces of that in the current mark.  This symbol is also sometimes called a "paraph."  According to Wikipedia, it is also sometimes called an "alinea" or "blind P," although even the OED doesn't seem to register those names, so I'm somewhat dubious.

The pilcrow is not to be confused with the pillcrow, a modern mythological bird that flies in your bathroom window at night and steals your medication, which is why you always seem to be short a couple of pills.