Spanish for "dough," masa is the traditional dough used to make corn tortillas. It's made with sun- or fire-dried corn kernels that have been cooked in limewater (water mixed with calcium oxide). After being cooked then soaked in limewater overnight, the wet corn is ground into masa. Masa harina (literally "dough flour") is flour made from dried masa.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.