Well-defined expression

A well-defined expression is like a recipe where every ingredient and every step is written down clearly so that anyone can follow it and get the same result. Just like how you need to follow the recipe to make a cake, a math problem also needs clear instructions. If a math problem is well-defined, it means that it has clear rules about what you can and cannot do, and everyone who tries to solve it will come up with the same answer. Think of it as playing a game with your friends - to play the game and have fun, everyone needs to know the same rules and play by those rules. That's what a well-defined expression means in math!