diff --git a/documentation/docs/introduction.md b/documentation/docs/introduction.md
index 012aea9..0d17c61 100644
--- a/documentation/docs/introduction.md
+++ b/documentation/docs/introduction.md
@@ -49,4 +49,13 @@ The inbox is responsible for getting messages from the transport and adding them
 
 1. Inbox gets messages from transport and adds to storage.
 2. In a separate process, Outbox will get messages and process them.
-3. Then processed messages are clean up.
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+3. Then processed messages are clean up.
+
+## Terms
+**`OutboxMessage`** - message that is added to the outbox and then converted to `TransportMessage`.
+
+**`TransportMessage`** - object transporting collections of messages of the same type (created on the basis of `OutboxMessages`). It is sent at least once.
+
+**`InboxMessage`** - created on the basis of `TransportMessage` and stored in the inbox. It is then processed.
+
+**`IdempotentInboxKey`** - incoming message identifier. It is needed for deduplication.
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