Hadoop is a highly available and secure enterprise data warehousing solution for big data. The document discusses that traditional data warehousing solutions like RDBMS are not suitable for big data due to challenges in storing and processing unstructured data at large scales. It then describes how Hadoop addresses these challenges through its scalable architecture and features like HDFS for storage, MapReduce for processing, and tools like Hive and HBase for data warehousing. The document also covers how Hadoop maintains high availability through techniques like secondary NameNodes and nameNode HA, and provides security through authorization and access controls.