Obligatory half-joke: The most commonly used definition deduced from reading online articles is, in terms of size "More data than naively fits into the memory

Obligatory half-joke: The most commonly used definition deduced from reading online articles is, in terms of size "More data than naively fits into the memory of my (midrange) laptop using a high overhead platform" or in terms of speed "More data per second than can be handled using the same naive database code we used in the 90s for our website's comment section"