AWS's pub/sub messaging service. It sends notifications to multiple subscribers (email, SMS, HTTP, SQS, Lambda) through topics. Fan-out model.
SAA-C03 uses this to decouple systems and send notifications. Key difference from SQS: SNS is pub/sub (one message to many); SQS is a queue (point-to-point with persistence). Fan-out pattern: SNS + multiple SQS queues.
Amazon SNS (Simple Notification Service) is a concept that can show up on the exam. Check the full guide to see it in context.