Open or free admission to all; hospitable welcome; free opportunity.
In modern diplomacy, opportunity for political and commercial intercourse open to all upon equal terms, esp. with reference to a nation whose policy is wholly or partially fixed by nations foreign to itself, or to territory newly acquired by a conquering nation. In this sense, often used adjectively, as, open-door system, open-door policy, etc.
You have to come to your closed doors before you get to your open doors... What if you knew you had to go through 32 closed doors before you got to your open door? Well, then you'd come to closed door number eight and you'd think, 'Great, I got another one out of the way'... Keep moving forward. Joel Osteen
Illegal immigration is crisis for our country. It is an open door for drugs, criminals, and potential terrorists to enter our country. It is straining our economy, adding costs to our judicial, healthcare, and education systems. Timothy Murphy
You can do anything you think you can. This knowledge is literally the gift of the gods, for through it you can solve every human problem. It should make of you an incurable optimist. It is the open door. Robert Collier