empty chair

Short definition from Wiktionary

Noun

1. 1
   A chair which is used to personify the client's object of conflict
2. 2
   A podium or chair left empty to highlight the absence of someone from a debate.
3. 3
   A position held by someone who is absent or otherwise ineffectual.

Verb

1. 1
   To draw attention to (a person who is absent from a debate) by providing an empty podium or chair which they would have occupied.