For the "International Standardization of Statistics Relating to Book Production and Periodicals", UNESCO defines a pamphlet as "a non-periodical printed publication of at least 5 but not more than 48 pages, exclusive of the cover pages, published in a particular country and made available to the public" and a book as "a non-periodical printed publication of at least 49 pages, exclusive of the cover pages". The UNESCO definitions are, however, only meant to be used for the particular purpose of drawing up their book production statistics.
The word pamphlet for a small work (opuscule) issued by itself without covers came into Middle English ca 1387 as pamphilet or panflet, generalized from a twelfth-century amatory comic poem with an old flavor, Pamphilus, seu de Amore ("Pamphilus: or, Concerning Love"), written in Latin. Pamphilus's name is derived from the Greek name Πάμφιλος, meaning "beloved of all". The poem was popular and widely copied and circulated on its own, forming a slim codex.
Its modern connotations of a tract concerning a contemporary issue was a product of the heated arguments leading to the English Civil War; this sense appeared in 1642. In some European languages other than English, this secondary connotation, of a disputatious tract, has come to the fore: compare libelle, from the Latin libellus, denoting a "little book".
Pamphlets can contain anything from information on kitchen appliances to medical information and religious treatises. Pamphlets are very important in marketing because they are cheap to produce and can be distributed easily to customers. Pamphlets have also long been an important tool of political protest and political campaigning for similar reasons.
A pamphleteer is a historical term for someone who produces or distributes pamphlets, especially for a political cause.