A more thorough explanation:

Definition: An opposer is someone who formally seeks to prevent the grant of a patent or the registration of a trademark in the field of intellectual property.

For example, if a company wants to register a trademark for their new product, an opposer may file a formal objection to the registration if they believe that the trademark is too similar to their own registered trademark or if they believe that the trademark is not distinctive enough.

In historical contexts, an opposer may have been referred to as an "apposer."

Overall, an opposer is someone who opposes the registration of a trademark or the grant of a patent in order to protect their own intellectual property rights.