In legal usage:

• The Appellee, or the opposing party, in an Appeal
• The Defendant in a proceeding commenced by a petition

In non-legal or informal usage:

• One who refutes or responds to a thesis or an argument in question
• In cross-cultural communication, the second person responding to the meaning or message from an original source which has been contextualised or decoded for the understanding of respondents as recipients or hearers of the message occurring from a different cultural context