Jiffy - a linux term for the tick of an interval timer - the smallest unit of time that the OS can track - e.g. on x86 architecture the standard interval timer ticks about every 0.45 microseconds, but the Linux OS typically loads an interval timer count such that it can control processes on a 10 millisecond software tick.
Jitter - when latency and/or timing of an operation or process changes with each iteration, this is jitter - i.e. when latency/timing is not constant. Jitter as a term can be used to describe many different types of operations or processes - e.g. execution jitter, period jitter, response jitter.
JTAG - Joint Test Applications Group, an IEEE committee that standardized the concept of boundary scan and the TAP (Test Access Port), which is used to verify integration of ASICS in a system (boundary scan), but is now also typically used in firmware development to control and single step a CPU by loading data and commands through the TAP with JTAG. JTAG includes the following signals: TDI (Test Data In), TDO (Test Data Out), TRST (Test Reset), Clock, Test Mode Set.