Substances which are repelled by external magnetic fields are called.

Hint: We know that electricity and magnetism are interrelated and the variation or the change in one of the two induces the other. We also know that magnetism can be observed in all the elements, hence it is classified into many sub categories, which are as discussed below.

Complete step-by-step solution:
We can define magnetism as the invisible force which aligns a freely suspended magnet along the earth’s magnetic field.
Magnetic properties of the metals can be of two types, either due to the effect of electric currents called electromagnetism or due to the spin property of the magnetic moments.
The magnetic moments are nothing but small magnets which are formed inside the atoms due the electrons which orbit in them, they can be classified as diamagnetism, paramagnetic, ferromagnetism, ferrimagnetism to name a few.
In diamagnetism, the materials tend to oppose the magnetic field or in other words repelled by the magnetic field.
In pragmatism, the electrons are mostly paired and thus their magnetic field cancels each other, however, there are a few unpaired electrons which align themselves in the presence of an external magnetic field.
In ferromagnetism, the electrons are mostly paired and a few unpaired electrons, however even in the absences of magnetic field, they try to attain low energy states by varying their electron orientation.
In ferrimagnetism, they are able to retain their magnetisation for a long duration even in the absence of an external magnetic field.
Thus, the substances which are repelled by external magnetic fields are called diamagnetic.

Note: The earth is the biggest magnet, which influences the magnets which are placed on the earth. This is also the reason why a freely suspended compass( which is also a magnet) comes to rest in the earth magnetic north and south. It is also worth noting that magnetic fields are invisible.