The strength of a covalent bond is defined by the energy required to break the bond, which is directly related to the stability the bond provides to the atoms involved. A stronger bond requires more energy to break, indicating that the atoms are held together very tightly. Typically, this energy is quantified in terms of bond dissociation energy, which measures how much energy is necessary to separate the bonded atoms in their gaseous state.
Factors such as the number of electrons involved and the types of elements can influence the strength, but they do not directly measure it. The distance between nuclei can also play a role in bond strength, as shorter distances often result in stronger attractions; however, it is ultimately the energy required to overcome these attractions that determines bond strength. Thus, the definition based on energy is the most fundamental measure of covalent bond strength.