Diamagnetic materials are those in which the individual atoms or molecules or ions do not possess any net magnet moment of their own.
When a sample of a diamagnetic material is placed in an external magnetic field of induction (B), a small magnetic moment is produced in each atom or molecule or ion proportional to B, but pointing in the opposite direction.
In other words, those materials which when placed in a magnetic field become weakly magnetized in a direction opposite to that of the applied field, are called as diamagnetic materials.
Example: bismuth, antimony, copper, gold, quartz, mercury etc.
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0. In case of any doubt, you can put your question in the comment section.