This lecture provides an introduction to big data, including its definition, sources, and key characteristics. Big data refers to extremely large data sets that are difficult to process using traditional methods. It comes from everywhere and is growing exponentially due to people, devices, sensors, and organizations constantly generating data. The key characteristics that define big data are volume, velocity, and variety. Volume refers to the extremely large size of data, velocity is the speed at which data is generated and processed, and variety means data comes in all types of formats. Examples are given of how various companies and domains work with big data and the challenges it poses in terms of capturing, storing, analyzing and visualizing such large and diverse datasets in a timely manner.