Turbofan is a type of airplane engine which is evolved from the turbojet Jet engine essentially by increasing the size of the first-stage compressor to the point where it acts as a ducted multiple thin propeller (or fan) blowing air past the "core" of the engine. This type of engine runs best from about 250 to 650 mph (400 to 1,000 km/h), which is why the turbofan is by far the most used type of engine for aviation use. The turbofan is also referred to as a high-bypass-ratio turbojet.
The bypass ratio (the ratio of bypassed air mass to combustor air mass) is an important parameter for turbofans. Early turbofans (and most modern jet fighter engines) are low-bypass turbofans. The Rolls-Royce Conway, the first turbofan, had a bypass ratio of 0.3, similar to the modern General Electric F404 fighter engine. In the late 1960s, high-bypass turbofans, which generally have bypass ratios of about 5, began to appear. These engines not only have much lower fuel consumption than low-bypass engines, but also could provide much higher thrust, which made wide-body aircraft much more practical. Today, almost all jet airliners are powered by high-bypass turbofans.
Turbofans (especially high bypass engines) are fairly quiet. The noise of a jet engine is strongly related to the velocity of the air coming out the exhaust. A turbofan has a larger mass flow of air for a given thrust than a turbojet, so the exhaust velocity will be slower and hence the turbofan engine will be quieter than an equivalent turbojet. Jet aircraft are often considered loud, but a conventional piston engine or a turboprop engine delivering the same power would be much louder. (NASA has a web page with details on jet noise.)
Other meanings
The Unicode standard includes a turbofan character, #274B, in the dingbats range. Its official name is "HEAVY EIGHT TEARDROP-SPOKED PROPELLER ASTERISK = turbofan". In appropriately-configured browsers, it should appear in quotes here: "❋";
