B

Explanation:
The term 'dielectric strength' refers to the maximum electric field that a dielectric material can withstand without breaking down or getting punctured. This characteristic is crucial in high-voltage applications, where dielectrics are used to insulate and separate conductive materials.

Step-by-step analysis:
1. **Dielectric Constant**: This is a measure of a material's ability to store electrical energy in an electric field, but it does not refer to the breakdown voltage or potential gradient.
2. **Dielectric Strength**: This is the correct term that defines the electric field strength at which a dielectric fails or becomes conductive. It is typically expressed in volts per unit thickness (e.g., kV/mm).
3. **Dielectric Resistance**: This describes the resistance of a dielectric material to electrical conduction, not the potential gradient that causes breakdown.
4. **Dielectric Number**: This term is not standard in the context of dielectrics and is not used to define the breakdown potential.
Therefore, the correct answer is Option B: Dielectric strength.