How does the Journey Builder API work? What is EventDefinitionKey and how is it used?

The Journey Builder REST API allows external systems to inject contacts into a journey via an API Entry Event. Steps:
1. In Journey Builder, create an API Event entry source — this generates an EventDefinitionKey.
2. Define an Entry Source Data Extension with the fields you want to pass into the journey.
3. Publish the journey.
4. Call POST /interaction/v1/events with body: {"ContactKey":"sub123","EventDefinitionKey":"APIEvent-abc123","Data":{"FirstName":"Jane","Email":"jane@example.com"}}.
Marketing Cloud processes the event asynchronously and enters the contact at the journey's entry point. Unlike triggered sends, Journey API entries are rate-limited and best used for behavioural events (cart abandonment, form fill) rather than high-volume batch sends.