Opposition systems offer third parties the opportunity to oppose the registration of a trademark within a certain period of time provided by the applicable law.

An opposition must allege at least one of the grounds for opposition – either absolute or relative – among those that are recognized under the applicable law.  Opposition procedures are closely related to the registration procedure.  They may be part of the initial registration procedure (pre-registration opposition) or directly follow the completion of the registration process (post-registration opposition).

Source: WIPO