A chair was a type of furniture on which an individual would sit.
A Slitheen disguised as Ambassador Rahnius blamed the noises made by him farting on the "very incriminating" chair he was sat on while visiting 13 Bannerman Road, suggesting to Sarah Jane Smith that she replace it. (TV: From Raxacoricofallapatorious with Love)
Inside the Byzantium, the Eleventh Doctor sat in a chair, which he referred to as a comfy chair, whilst talking to Angel Bob; the Doctor took great amusement when he got Angel Bob to say "comfy chairs". (TV: Flesh and Stone)
Inside the Minotaur's prison ship, Rita threatened the Eleventh Doctor with a chair leg. The Doctor claimed he had never been threatened with a chair leg before, but he quickly corrected himself by saying, "No, hang on. I tell a lie." (TV: The God Complex)
When Davros was dying he requested the Twelfth Doctor speak to him one last time. (TV: The Magician's Apprentice) When the Doctor arrived Davros had the Daleks procure "the only other chair on Skaro" (the other being his own) for the Time Lord insisting that his foe should feel grateful and privileged due to the near impossibility of finding it and told him not to get up. The Doctor quickly picked himself up and proceeded to ignore the chair. (TV: The Witch's Familiar)
After realising the castle inside his confession dial was in the sea, the Twelfth Doctor threw a stool out of the castle window to determine the wind resistance and how many seconds it took to hit the water so he could see if he could escape the Veil by jumping out himself. The room reset itself in each loop of the Doctor dying and being brought back to life, meaning the stool returned to the room. (TV: Heaven Sent)