Obesity is defined as weight that is:

Obesity is characterized as having a body weight that is higher than what is considered healthy for a person's height. This definition is often measured using the Body Mass Index (BMI), which factors in both weight and height to determine whether an individual has a healthy weight, is underweight, at a normal weight, overweight, or obese. The emphasis on height relative to weight is crucial because individuals of different heights can have vastly different healthy weight ranges.

In this context, the correct choice highlights that obesity is not simply about having excess weight but rather about having a weight that poses potential health risks due to being above what is deemed healthy for one's specific height. This understanding helps healthcare professionals identify and address the risks associated with obesity, such as cardiovascular disease, diabetes, and other health complications, further emphasizing the importance of maintaining a healthy weight tailored to individual height standards.

The other options do not accurately reflect the clinical definition of obesity. For instance, suggesting that obesity is 'optimal for health' or 'ideal for athletic performance' misrepresents the health risks associated with excessive weight. Similarly, defining obesity as 'below average for age' is not relevant, since being underweight may lead to health issues, just as being overweight can.