What is the counterpart to 'reply'?

Solution 1:

It's usually called the parent message, using an analogy between a discussion thread and genealogic tree. At least in technical or scientific circles, this is well understood.

You can also call it your original message, your initial message, your query or your request (if appropriate).

Solution 2:

You might use the word prompt in either its noun or verb form, as in, "What was the prompt for this reply?" or "What prompted this reply?" This would be--I think--a non-technical, not jargonized, word for the concept you are trying to articulate.

Prompt -

: to cause (someone) to do something

: to be the cause of (something)

: to say (something that encourages a person to talk)

http://www.merriam-webster.com/dictionary/prompt

Solution 3:

If there is a single message and a single response, then the first is referred to as the original message.

If this a mailing list discussion or forum discussion, then the first message can be referred to as the original post or OP (OP can also refer to person who submitted the first post: the original poster). The collection of original post and responses are referred to as a thread. If you were unable to see the messages or posts before a certain response, you might say you didn't see the original post or you couldn't see the entire thread.

Solution 4:

The answer is "message" and I will explain why.

This is because reply is not a noun with a specific and direct relative. It is a very general word.

Here are some nouns and their counterparts:

Parent - Children

Question - Answer

Ancestor - Descendant

The difference between children, answer and descendant is that each of them has a direct relation to their precursor. To be a child can only be an offspring of a parent, an answer can only denote the existence of a question and a descendant will have to have ancestors. It's basically law.

Reply has no specific relative because an answer is also a reply but a reply may or may not be an answer!

"Reply" is very general and the only other general word I can think of that is most relevant is "message".

For and against argument:

A parent denotes a child and a child denotes a parent. However, a question does not denote an answer but an answer denotes a question.

Hence, the answer to your question will have to be based on "reply" in order to be 100% accurate with the other definitions of counterparts - the existent of A means also the existence of B but may not be vice versa, yet they are still considered counterparts.

A reply absolutely denotes a message.