The word "insubordination" is from Latin and means "not being subordinate." It is considered a very serious offense. But it is not simple disobedience or even a failure to follow legitimate orders. Insubordination rises to a higher level. Since it is so serious an infraction, it takes a lot more obstinacy to be insubordinate!
Normally, insubordination requires that a direct order be given and the order be reasonable, legal, and not in violation of contracts, work rules, etc., and that the worker refuses to obey the order, and then be directed to do so after the worker has explained his reasons for refusal.
Here are some references: