New York Style
New York Style is a format modifier, used notably at the inaugural Pro Tour event, Pro Tour New York 1996, and occasionally at the Magic Invitational and similar events.
Original New York Format
Pro Tour New York was played shortly after the release of Homelands, infamously one of the weakest sets ever printed. While the actual decision process is unknown, it is widely believed that WotC found it embarrassing that it was likely that few Homelands cards would be played at the Pro Tour and likely none would be in evidence in the decklists published for the top tables. To avoid this, or possibly for some other mysterious reason, PT New York was not just Standard as expected, but had the additional restriction that every decklist had to contain at least 5 cards from each set which had been printed to date. (Alpha/Beta/Unlimited were considered a single set, and Revised was excluded.)
Derivative Format Modifier
A New York Style format has some list of permissible sets, usually but not always equivalent to an existing normal format, with the requirement that either 4 or 5 cards must be included from each permissible set. Sometimes these must be in the main deck; others, either main deck or sideboard.
Notable Instances
The Magic Invitational used New York Style on two occasions[1]: