Section 16. The term ''subversive organization'' as used in sections seventeen, eighteen, nineteen, twenty-one, twenty-two and twenty-three of this chapter shall mean any form of association of three or more persons, however named or characterized, and by whatever legal or non-legal entity or non-entity it be established, and whether incorporated or otherwise for the common purpose of advocating, advising, counseling or inciting the overthrow by force or violence, or by other unlawful means, of the government of the commonwealth or of the United States.