The term "focus" in relation to an earthquake refers to the point beneath the Earth`s surface where the earthquake originates. It is also known as the hypocentre. The hypocentre is the exact location where the seismic energy is released, causing the ground to shake.

Option 1: Hypocentre - This is the correct answer. It is the other term used for the focus of an earthquake.

Option 2: Epicentre - The epicentre is the point on the Earth`s surface directly above the focus or hypocentre of an earthquake. It is the location where the earthquake`s effects, such as shaking and damage, are usually the strongest.

Option 3: Isocentre - This term is not commonly used in relation to earthquakes. It does not refer to the focus or hypocentre.

Option 4: Principal Point - This term is also not used in relation to earthquakes. It does not refer to the focus or hypocentre.

Therefore, the correct answer is option 1, hypocentre, which is the other term used for "focus" in relation to an earthquake.