I usually change "one machine" with "my notebook" and "fit" with "can analyze". It is big data if I cannot analyze it using my notebook. So it depends both on the size of the data (a petabyte is big data), the performance requirements (10GB/s is big data even if I keep 1 minute of data in the system) and also depends on the kind of analysis (doing TSP on a 1000000-node graph is big data, even if it fits my notebook memory).

I also define "small data" as anything that can be analyzed using Excel.

It is usually dramatic enough to get buy-in :).