General synonyms:
* Stateless
* Unconnected
* Non-connected
* Independent
* Autonomous
* Disassociated
* Detached
Synonyms in the context of networking:
* Datagram
* Packet-switched
* Message-oriented
* Unreliable (in the sense that there is no guarantee of delivery)
The best synonym will depend on the specific situation. For example, if you are talking about a computer network, "packet-switched" would be a more specific and accurate synonym than "stateless."