Definition
The forward force produced by the rearward expulsion of exhaust gases from a turbine engine. In a turboprop, jet thrust is the small portion of total thrust contributed by the exhaust stream, while the majority of thrust comes from the propeller driven by the turbine.
Plain English
The push you get from hot exhaust gases blasting out the back of the engine. In a turboprop, the propeller does most of the pushing, but the exhaust adds a little extra forward shove on top of that.
Context Anchor
Seen in turboprop engine discussions when comparing the thrust made by the propeller with the smaller thrust made by the engine exhaust.
Derivation
"Jet" comes from the French jeter, meaning to throw or hurl. The exhaust gas is literally thrown rearward, and by Newton's third law, the engine is pushed forward. Knowing this anchors the idea that any thrust labeled "jet" comes from gases being hurled out the back.
Why Pilots Care
It adds to overall thrust output and becomes more significant as airspeed increases and propeller efficiency drops.
Intuition Check
Do not read “jet thrust” as meaning the airplane is simply a jet airplane. Here it means the part of thrust made by exhaust gas moving rearward, separate from the thrust made by the propeller.
Example Sentence 1
In a typical turboprop, about 90 percent of the forward force comes from the propeller and only about 10 percent from jet thrust.
Example Sentence 2
At cruise speed the contribution of jet thrust becomes a larger part of total forward force.