from The American Heritage® Dictionary of the English Language, 4th Edition
- n.pl. A volume or section of a telephone directory that lists businesses, services, or products alphabetically according to field.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. a telephone directory that lists commercial organizations
So called because they are usually printed on yellow paper.(American Heritage® Dictionary of the English Language, Fourth Edition)
From such directories being printed on yellow paper (Wiktionary)