Terminology Question

Speaking of definitions, what term are folks using to describe the
collection of messages that occur when an operation is invoked.  For the
typical input/output operations you would have 2, a request and a
response. In a one-way you would have 1, in a solicit response you would
have n etc.  

What moniker would y'all recommend for this collection:

1.  Exchange
2.  Conversation
3. ???

Something that indicates the fact that these messages are part of the
same operation is the critical part for me.

Thanks.

Jim Murphy
Mindreef, Inc.
http://www.mindreef.com