•  A written conversation on a particular topic in a larger group discussion.

Have you ever sent an e-mail that referred to previous conversation?

E-mail can be used to "thread discussions." Threaded discussion include both e-mail and non-email messages.  

• Non-e-mail messages are prior conversations or discussions, whether face-to-face or via telephone, and includes business processes and procedures, both formal and informal.
• Threaded discussions include a face-to-face conversation that occurred with a third party before the e-mail message was received.   
• By bringing together the mail thread and an earlier discussion the participants use e-mail to  resolve a task from an earlier conversation/message before responding to the e-mail sender and third party.
• E-mail can be used to current committee workload and allocate new tasks. In this manner the e-mail message further clarifies and confirms the initial discussion.