What is the difference between a high bypass turbofan and a low bypass turbofan?
The first turbofan, the RR Conway, had just a bypass ratio of 0.25. It was designed for installation into the wing root of airplanes like the HP Victor bomber or the Vickers V-1000 transport project, so RR strived to keep the diameter small. Of course, back then nobody distinguished low- and high-bypass engines. The P&amp;W JT-3D, which powered most of the second generation of jet airliners (Boeing 707 and DC-8), had a bypass ratio of less than 1.5.

But when the new generation of engines (GE CF-6, JT-9D) with bypass ratios of around 5 were developed for the first generation of wide body jets, marketing needed a term to make clear that this was a new generation of engines, and the high-bypass ratio engine category was born. Generally, the threshold is around 5, but note that some CF-6 versions have bypass ratios as low as 4.24 and still fall into the high bypass ratio category. Therefore, Wikipedia sets the limit at a bypass ratio of 4.

There is no technical difference between a low- and a high-bypass engine, and the limit is arbitrary. The jump in bypass ratio from 1.5 to 5 brought significant improvements in specific fuel consumption and noise reduction, but part of the technical progress was also from better materials and aerodynamics, which allowed higher compression ratios and turbine entry temperatures.

Now, almost 50 years later, we see the first engines with bypass ratios above 10 entering service. Designs are being readied with bypass ratios of 12, so marketing will need a new word to describe those creations. With "ultra high bypass" already taken by the unducted fan crowd, the search is on.