A paper size used in the United States, measuring 8.5 inches by 14 inches. Slightly longer than letter.
Lawyers may have been the first to use paper with dimensions close to this, hence the name. It certainly was in common legal usage until recently. Ironically, most jurisdictions accept only letter sized papers for filings and depositions. In those jurisdictions, it is against the law to use legal-sized paper for purposes of law.
A running joke: lawyers use legal size because they need 14 inches to say what ordinary folks can fit in 11. (Cecil Adams, "The Straight Dope")