Here’s a definition recently discussed by the CFTC. High Frequency Trading (HFT) is a form of automated trading that employs:
a) algorithms for decision-making, order initiation, generation, routing or execution;
b) low latency technology that is designed to minimize response times, including proximity or co-location services;
c) high speed connections to markets or order entry;
d) recurring high message rates (orders, quotes or cancellations), using one or more forms of objective measurement, including cancel-to-fill ratios, participant-to-market message ratios, participant-to-market trade volume ratios.