What term describes a fraction that cannot be simplified further?

The term that describes a fraction which cannot be simplified further is "lowest terms." When a fraction is in lowest terms, it means that the numerator and the denominator have no common factors other than one. This simplification ensures that the fraction is presented in its simplest and most understandable form, making calculations and comparisons easier.

In contrast, an improper fraction is one where the numerator is greater than or equal to the denominator, while a proper fraction has a numerator that is less than the denominator. These terms relate to the size of the fractions rather than their simplification status. The term "reciprocal" refers to the fraction obtained by flipping the numerator and denominator, which does not address whether a fraction can be simplified. Therefore, the correct answer effectively captures the concept of simplification in fractions.