An S3 bucket is a storage container used in Amazon S3 (Simple Storage Service) to hold data in the cloud. It allows users to store and retrieve any amount of data at any time from anywhere on the web. Each bucket can contain an unlimited number of files, known as objects, which can be organized using a hierarchical structure. Users can manage access permissions for their S3 buckets, ensuring that only authorized individuals or applications can view or modify the stored data. This makes S3 buckets a popular choice for businesses and developers looking to securely store backups, media files, and other important information.