Fantastic you know the dictionary definition of a diet.Also you're wrong about diet as it refers to both the type and quantity of food you put in your body. A calorie in regard to food is a measure of the energy a food provides you. When someone says to go on a weight loss diet it means to modify your diet to lower calories either by lowering the amounts of food you currently eat or by eating food that is lower in calories at the same quantity most times doing both.
But in a practical manner…
Diet as in calories is typically short term
Diet as in the types of food you eat is a lifetime.
Bc unless you’re genuinely fat, an adult’s weight mostly stays the same. The avg person when they say they ‘need to lose weight’ that’s not what they actually mean. What they mean is they need to get in shape, they need to tone up. The weight # is not the real issue
And even if it is, ok you’ve lost the weight now what?!? You counting calories the rest of your life? Of course not. What you really do is eat the right foods, know when to eat em, foods you no longer buy, etc. That’s the REAL meaning of a diet. It’s a lifestyle, not a 1 off endeavor.