Broadcast and Campaign are interchangeably used to refer to one-off messages sent to a group of contacts. However, Campaigns don’t necessarily have to be through messaging or one-off. Campaigns can be through other channels like forms, surveys or ads.
Other definitions
Macmillian Dictionary

A series of things such as television advertisements or posters that try to persuade people to buy a product.

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Any distributed content, that's created and measured in Mailchimp, including regular emails, automations, landing pages, and ads.

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