LCM - Least Common Multiple, the LCM is the smallest number which is also multiple of 2 different numbers - e.g. given x=3, y=5, the LCM(x,y)=15. This concept is key to periodic service analysis in real-time theory because it is necessary to diagram service times over the LCM of all periods in order to fully analyze timing demands upon a resource.
Least Laxity First (LLF) - a dynamic priority policy where services on the ready queue are assigned higher priority if their laxity is the least (where laxity is the time difference between their deadline and remaining computation time) - this requires the scheduler to know all outstanding service request times, their deadlines, the current time, remaining computation time for all services, and to re-assign priorities to all services on every preemption. Estimating remaining computation time for each service can be difficult and typically requires a worst-case approximation.
LED - Light Emitting Diode, a device typically used to provide visual I/O and status for an embedded system.
Lehoczky, Sha, and Ding Theorem - If a set of services can be scheduled over the period of the longest period service after a critical instant, then the system is feasible (i.e. is guaranteed not to miss a deadline in the future).