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Deleting a chat clears your conversations with a contact and deletes your chat with them. You won't be able to find the chat in the Chat tab. If you want to talk to a contact whose chat you've deleted, you'll have to start a new conversation with them. 

Clearing and deleting old conversations will free up space on your device. After your first cleanup, repeat the process every few months to keep your WhatsApp light and airy.