What big data means
The definition most cited in technical circles comes from NIST, the US standards institute. In its 2019 interoperability framework, it describes big data as extensive datasets, characterised mainly by volume, variety, velocity or variability, that require a scalable architecture for efficient storage, manipulation and analysis.
The second half of that definition is the important part. Big data is more than "a lot of data": the size, the pace at which it arrives or the mix of formats forces a change of tool. A spreadsheet or a database on a single server stops being enough, and the work is spread across many machines.
The four characteristics are easier to see with examples. Volume: the log files of a large web server, with millions of lines a day. Velocity: the clicks in an online shop that have to be processed as they happen. Variety: tables alongside text, images or machine logs. Variability: data whose meaning or rhythm shifts over time, such as a website's traffic before and after a Google update.