San An·to·ni·o (sān ān-tō'nē-ō') A city of south-central Texas southwest of Austin on the San Antonio River, flowing about 322 km (200 mi) southeast to San Antonio Bay on the Gulf of Mexico. The city was founded as a Franciscan mission in 1718 and is the site of the Alamo, which was besieged and captured by Mexican forces in February-March 1836. It has several military bases. Population: 1,300,000.