seconder       
noun [ C ]  politics specialized uk/ˈsek.ən.dər/ us/ˈsek.ən.dɚ/
 (会议中对提名、动议等的)附议者,赞成者
someone who makes a formal statement of support for a suggestion made by someone else during a meeting, so that there can be a discussion or vote
  There was no seconder for the motion so it could not be debated. 该议案没有附议者,因此无法进行辩论。 
 See: second
 - More examples
 - His proposal failed to find a seconder.
 - We have a motion proposed by Mr Connon. Is there a seconder?
 - The names of the proposer and seconder will not be published and will remain confidential to the scrutineers.
 - Only corporate members may participate in elections, either as candidate, proposer, seconder or voter.
 - All the candidates had to be nominated with proposers and seconders and they all have election agents.