Definitions for "Open proxy"
proxy server that offers unrestricted, anonymous access to anyone and everyone. Usually, in the context of spam, this term refers to proxies that allow the user to create arbitrary TCP connections. SOCKS proxies and HTTP proxies with SSL support meet that requirement. Other types of proxies can be "open" too, but they are less useful to spammers.
a computer that has essentially been hacked into and turned into a spam delivery mechanism, usually without the owner's knowledge or consent
a mis-configured proxy server through which an unauthorized user can connect to the Internet