Charta is Latin for paper (originally paper made from papyrus, but paper none the less). Chartula is the diminutive form of charta, and refers to a small piece of paper or a small bit of writing. We get the word charter from chartula , along with the lesser known word 'chartulary' (or 'cartulary'), a keeper of archives.* (We also get the words card and chart from charta, along with the obvious Magna Charta).
In the field of medicine, 'chartula' was used to refer to a folded piece of paper holding (usually) a single dose of a medicinal powder. This may also be referred to as a powder paper. We don't usually package our medicines this way any more, so while this word is listed in most larger dictionaries, especially medical dictionaries, it is mostly used in historical fiction and fantasy novels (wizards and witches may use chartulae for their powders).
Pronounced kahr-teh-leh or karh-cheh-leh. Plural is chartulae.
* Do you actually care about this, or do you just automatically read every footnote that someone throws at you? Well, for those of you who are interested: chartulary can refer to either the archivist or the archives, and comes from the Latin chartularius, meaning roughly 'that which has to do with little bits of paper'. Yay Latin!