The pilcrow (¶), also called the paragraph mark, paragraph sign, paraph, alinea, or blind P, is a typographical character for individual paragraphs.
The pilcrow can be used as an indent for separate paragraphs or to designate a new paragraph in one long piece of copy. The pilcrow was a type of rubrication used in the Middle Ages to mark a new train of thought, before the convention of visually discrete paragraphs was commonplace
The derivation of its name is as complex as its form. It originally comes from the Greek paragraphos(para, “beside” and graphein, “to write”), which led to the Old French paragraph, which evolved into pelagraphe and then pelagreffe. Somehow, the word transformed into the Middle English pylcrafte and eventually became the “pilcrow.”
Excerpt of a page from Villanova, Rudimenta Grammaticæ showing several pilcrow signs in the form common at that time, circa 1500 (image: Wikimedia commons).
(Thanks to Mish for this word.)