In pilot jargon, the one bolt on an aircraft which mythically holds the whole contraption together. May also refer to any single fastener which actually does hold together an entire subassembly. It doubtless gets its name from what the pilot would find him- or herself shouting if it should fail.
Most aircraft do have fixtures which must be inspected during the preflight to see that they have not exceeded some stress tolerance, and the Jesus bolt is often one such item. These usually take the form of a nut and bolt with a pin or safety wire inserted through both, which would bend or break if overstressed.
On T-tail Piper airplanes, for instance, the part most often called the "Jesus bolt" is located just forward of the elevator where it intersects the rudder. Some other aircraft even have a little Lexan window through which the Jesus bolt, or its equivalent, may be viewed.
In the world of helicopters the term has another sense: for them, it is a fastener that holds on the rotor blade – or even, in a radio controlled model helicopter, the entire rotor assembly. According to this interpretation, they call it a Jesus bolt because the operator must have faith in it. (thanks to Alexa for this tidbit)