I learned the following definitions. (Assume all numbers are non-negative.)

Proper Fraction: A ratio where the numerator is smaller than the denominator

Improper Fraction: A ratio where the numerator is larger than or equal to the denominator

In other words, a proper fraction's value is less than 1, while an improper fraction's value is 1 or more.

All Whole numbers larger than zero have a value that's 1 or more, so they're all improper fractions when written as a ratio.