Big Data refers to extremely large and complex datasets that exceed the processing capacity of traditional database systems, requiring distributed storage and parallel processing frameworks. Originating from the need to handle web-scale data from search engines and social networks, Big Data is characterized by the Five Vs: volume (petabytes to exabytes), velocity (real-time ingestion), variety (structured, semi-structured, unstructured), veracity (quality and trustworthiness), and value (actionable insights). The ecosystem spans batch and stream processing, NoSQL databases, cloud platforms, and machine learning frameworks. Understanding Big Data means mastering not just storage and computation, but also data governance, quality, security, and the trade-offs between consistency, availability, and partition tolerance that define distributed systems.