Nachos are tortilla chips arranged on an oven-proof platter or dish, topped with cheese and chile and broiled/grilled.
Other toppings usually include tomato.
Some more mediocre versions top them with chili con carne, making them heavy and gloopy.
"Nacho" was actually the middle name of the cook who invented Nachos; his name was Ignacio "Nacho" Anaya. In 1943, he was working in the Victory Club restaurant in Piedras Negras, Mexico (across the border from Eagle Pass, Texas). He had to cook for some officers' wives from Fort Duncan Air Base in Texas, who had driven across the border to eat at the restaurant at a time when the kitchen supplies were low. He actually used a tostado -- a whole, fried corn tortilla -- instead of tortilla chips. He grated some American cheddar on them, melted it under a grill (a Salamander), and when melted, topped it with slices of jalapenos. Every October there is still a Nachos competition in Piedras Negras
In 1977, a man named Frank Liberto changed the recipe, to use tortilla chips to make it easier to eat as snack food in Arlington Stadium in Arlington, Texas. Howard Cosell, a famous sports reporter, liked both the food and the name, and talked them up in his reporting, and the dish became very popular.