A "compellable witness" refers to a person who can be legally forced or required to give testimony or evidence in a court of law. They cannot refuse to testify when summoned or called upon by the court. Full definition
50 Spouses are competent and compellable witnesses against each other in proceedings under this Act. (ontario.ca)
(2) Neither the Fairness Commissioner nor anyone employed in the Office of the Fairness Commissioner is a competent or compellable witness in a civil proceeding outside this Code in connection with anything done under this Code. (ontario.ca)
While there must obviously be sanctions (including potential imprisonment) for refusing to testify, they should only be imposed on compellable witnesses who willfully defy judicial authority, and only in accordance with the ordinary rules of criminal procedure and evidence. (ualbertalaw.typepad.com)