The apophysis is a normal secondary ossification centre that is located in the non-weight-bearing part of the bone and eventually fuses with it over time (most of the apophyses fuse during the 2nd decade of life, but this process can be delayed, especially in female athletes).
It is a growth plate that isn't involved in the linear growth of a bone.
The apophysis is a site of tendon or ligament attachment, as compared to the epiphysis which contributes to a joint, and for that reason, it is also called 'traction epiphysis'.
When unfused, apophyses can easily be mistaken for fractures.
In skeletally immature patients the physeal cartilage is weaker than adjacent bone, ligaments and tendons. therefore, it is most prone to injury in this age group. Apophyseal injury can occur in the setting of acute trauma, which often leads to apophyseal avulsion, or in chronic overuse, which is associated with apophyseal stress injury.
Given the similar development with epiphyses, primary bone lesions that are typically located in the epiphyses can also occur in apophyses.