In this article, unless the context otherwise requires, "hospital service corporations", "medical service corporations", "dental service corporations", "optometric service corporations" and "hospital, medical, dental and optometric service corporations" mean corporations organized under the laws of this state for the purpose of establishing, maintaining and operating nonprofit hospital service or medical or dental or optometric service plans,  or a combination of such plans, whereby hospital, medical or dental or optometric service may be provided by hospitals, which within the meaning of this article may include extended care facilities and home health agencies, or by physicians, which within the meaning of this article may include professional and technical personnel under the direction of a physician, or by podiatrists, or by dentists which may include those engaged in the general practice of dentistry as well as the specialized or restricted practice of dentistry, or by optometrists which may include those engaged in the general practice of optometry as well as the specialized or restricted practice of optometry, with which the corporations have contracted for such purpose, to such of the public as become subscribers to the corporations under contracts that entitle each subscriber to certain hospital, medical, dental or optometric service, or in the case of hospital service corporations or medical service corporations, all such services, or whereby as operating expense or refunds, payments may be made to subscribers with respect to any such service that is rendered by a hospital, physician, podiatrist, dentist or optometrist with which the corporations have not so contracted.