Wiktionary
n. Paper treated so as to change color on exposure to heat.
Wikipedia
Thermal paper is a special fine paper that is coated with a chemical (containing BPA) that changes color when exposed to heat. It is used in thermal printers and particularly in inexpensive or lightweight devices such as adding machines, cash registers, and credit card terminals.
The surface of the paper is coated with a solid-state mixture of a dye and a suitable matrix; a combination of a fluoran leuco dye as an example. When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its colored form, and the changed form is then conserved in a metastable state when the matrix solidifies back quickly enough. The reactant acid in thermal paper is often bisphenol A (BPA).
Usually, the coating will turn black when heated, but coatings that turn blue or red are sometimes used. While an open heat source, such as a flame, can discolor the paper, a fingernail swiped quickly across the paper will also generate enough heat from friction to produce a mark.
Multicolor thermal paper first became available in the early 1990s with the introduction of the Fuji Thermo-Autochrome (TA) system. This was followed in the early 2000s by the development of the Polaroid "zero-ink" (ZINK) system. Both of these methods rely on multi-layer coatings with three separate colorizing layers, but different methods are used for independent activation of each layer.
Usage examples of "thermal paper".
The slick thermal paper was covered mainly with gibberish, some discrete but disordered letters, and even more black-and-white hash from the random noise, but perhaps as much as twenty percent was legible English, including two complete sentences and one full paragraph.
The red message light was blinking on the latter, and a couple of flimsy sheets of thermal paper were in the tray of the former.
Printed on that thermal paper photocopies used in those days, it is more yellowed and faded now.