from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Designating sound recording equipment or sound recordings that use magnetic tape which must be threaded through the equipment and onto an empty reel.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A machine that records sound onto, or plays sound from, tape wound on one reel, by feeding it through to another reel.
Originally descriptive, used in reel-to-reel recorder and like phrases, then shortened. (Wiktionary)