Onionskin or onion skin is a thin, light-weight, strong, often translucent paper

Paper

Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

. It was usually used with carbon paper

Carbon paper

Carbon paper is paper coated on one side with a layer of a loosely bound dry ink or pigmented coating, usually bound with wax. It is used for making one or more copies simultaneous with the creation of an original document...

 for typing duplicates in a typewriter

Typewriter

A typewriter is a mechanical or electromechanical device with keys that, when pressed, cause characters to be printed on a medium, usually paper. Typically one character is printed per keypress, and the machine prints the characters by making ink impressions of type elements similar to the pieces...

, for permanent records where low bulk was important, or for airmail

Airmail

Airmail is mail that is transported by aircraft. It typically arrives more quickly than surface mail, and usually costs more to send...

 correspondence. It typically has a 9 pound basis weight, and may be white or canary colored.

In the typewriter era, onion skin often had a deeply-textured cockle finish which allowed for easier erasure of typing mistakes, but other glazed and unglazed finishes were also available then and may be more common today.

Onionskin paper is relatively durable and lightweight due to its high content of cotton fibers. Because of these attributes and its crispness when folding, onionskin paper is one of the best papers to use for advanced paper airplanes. Paper airplanes made from onionskin paper tend to fly very well due to its low weight and high integrity once folded.