A tostada is a cooked tortilla that is further cooked to make it crispy.
The tortilla can be toasted or deep fried. It can be cooked flat as it is, or it can have been moulded into a bowl shape to act as a container for a food item. Corn tortillas are generally used rather than wheat ones.
Historically, tortillas were dry-toasted in a pan (a comal) without any oil. Now, most people in practice fry the tortillas in a small amount of oil in a pan.
You can also deep fry the tortillas for about 30 seconds per side in very hot oil, then drain. Or to bake, arrange the tortillas in a single layer on a cookie sheet, spray with oil, and put in a hot oven for about 3 minutes.
Tostada also refers to a (food) dish made with a tostada. Appetizer tostadas are often flat, covered with a layer of refried beans or meat to act as an “adhesive” anchor, and then covered with other toppings on top of that.
In Tex-Mex cuisine, the term tostada can refer to tortilla chips.
Toasting a tortilla gave it shelf life, so that it could be stored for weeks. Making Tostadas was also a way to use up stale (though not mouldy) tortillas.
“Tostada” means “toasted.”
Walsh, Robb. The Unconquerable Tostada – Zapotec food history. Natural History. April 1999.