Heat a large skillet over medium heat. Add 1 pound lean ground beef and cook, breaking it into small pieces with a spatula, until the beef is no longer pink.
Stir in 2 tablespoons taco seasoning mix and 1/2 cup water. Bring to a simmer and cook, stirring occasionally, until the sauce thickens, about 5 minutes.
Remove the skillet from the heat and let the meat cool for about 5 minutes.
While the meat cools, combine 4 cups chopped romaine lettuce, 1 medium diced tomato, and 3/4 cup shredded cheddar cheese in a large bowl.
Slightly crush 1 1/2 cups nacho cheese–flavored tortilla chips (such as Doritos) and set them aside.
Add the cooled taco-seasoned beef to the bowl with the lettuce mixture and toss gently to combine.
Just before serving, sprinkle the slightly crushed tortilla chips over the salad and drizzle 1/2 cup light salad dressing over the top. Toss gently once more to distribute the dressing and chips.