A ceramic seal assembly (10) for electrically insulating and hermetically securing a terminal to a housing through which the terminal extends. The housing contains a fluid at a high pressure and an electrical device to which the terminal is connected. An adapter (40) is provided for hermetically securing the seal assembly (10) to the housing. The adapter comprises a generally tubular member defining two end portions and a collar (15) secured to one of the two end portions. The seal assembly (10) and housing are hermetically secured to the tubular member at separate locations. The seal assembly (10) contains at least one annular ceramic member (11, 12, 13) and is oriented such that the high pressure fluid contained within the housing causes the annular ceramic member (11, 12, 13) to be in compression and thereby maintain a seal between the terminal and the housing.