A court which is session but not hearing any viva voce evidence, or otherwise in trial.
Historically, a judge in his private office (i.e. chambers) who would meet with the lawyers and resolve housekeeping matters so that the trial could proceed. That evolved, for reasons of transparency, to a judge sitting in a publicly-accessible courtroom and hearing procedural matters, but kep the term chambers.
Notably, a judge is not gowned when sitting in chambers, nor are the lawyers. Evidence, if any, is not given by oral testimony but by affidavit and generally, only lawyers or self-represented parties may speak and then only as to their matter before the Court.
Of late, some jurisdictions have taken to refer to chambers as any court sitting and dealing either with procedural matters only or any application which is heard by affidavit evidence.
In some courts, such as smaller or isolated registries of the Provincial Court of British Columbia, matters such as case conferences are hosted by a judge in his or her private office, though some judges prefer to keep lawyers out of their offices, and away from the private sanctuary of judges, as much as possible.
Still, some judges may ask to see the lawyers in "their chambers" for some off-the-record conversation inappropriate for open court but necessarily, for the purposes of justice, in camera and in confidence at that time.
On rarer occasions, a judge may ask to see a lawyer in chambers, referring again to his or her private office, to discuss some unrelated, usually personal or social matter. As a younger lawyer, I almost fainted when a judge asked me into his chambers right after dismissing my client's application. The apprehension was for naught as the judge merely wanted to welcome me to Victoria and ascertain if I had any political aspirations!
Some judges call the lawyers into their offices regularly during the course of a trial in an attempt to keep the hearing moving along.
On other occasions, where the safety of a person might be at risk if a necessary judicial consultation were held in open court, I have been called into a judge's private chambers to be advised as to the terms of her pending and controversial decision.