Yarn is a long continuous length of interlocked fibers, suitable for use in the production of textiles, sewing, knitting and weaving. Yarn can be made from any number of artificial or natural fibres. Very thin yarn is referred to as thread. Yarns are made up of any number of plys, each ply being a single thread these threads being twisted (plied) together to make the final yarn.

In some cases, thread may be monofilament, in which case it is a single fiber. The only natural fiber that is counted as monofilament is silk.

Yarn is manufactured by either a spinning or air texturizing (commonly referred to as taslanizing) process.

Yarn manufacturing was one of the very first processes that was industrialized.

Yarn is usually measured by weight. In the United States, balls of yarn are usually sold in three-ounce, four-ounce, six-ounce, and eight-ounce skeins. In Europe the units used by textile engineers is often tex. This is grams per kilometer. Many other units have been used during the last centuries each industry creating its own for internal purposes and these escaping into the public domain.

A long, rambling and involved story or a very lengthy joke with the main source of humour in the punchline is also known as a yarn. Also known as a Shaggy dog story or a campfire yarn.