- RFC2616 calls it an Exchange.
- Wireshark and HTTPNetworkSniffer call it a Request/Response.
- Fiddler calls it a Session.
- Charles calls it a Sequence.
- HTTP Scoop calls it a Conversation.
- Other vocabulary includes: Message, Transaction, Communication.
I would go for Exchange or RequestResponse.
I also went to name it Operation in my code as I would queue Operations, flush Operations, pause or resume Operations.