For a number to have an odd number of positive factors, it must be a perfect square.
The perfect squares less than $100$ are $1, 4, 9, 16, 25, 36, 49, 64, 81$.
The greatest perfect square less than $100$ is $81$, so the greatest whole number less than $100$ that has an odd number of positive factors is $\boxed{81}$.
The answer is: 81
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Name the greatest whole number less than $100$ that has an odd number of positive factors.

### Answer:
