Elmore delay
Beginner
A quick way to estimate how long a signal takes to travel along a wire. Delay grows quickly with length: a wire twice as long takes about four times as long.
Novice
A quick estimate of signal delay through a network of wires: for each piece of wire, multiply its resistance by all the capacitance beyond it, and add up the results. Because both grow with length, an unbuffered wire’s delay grows roughly with the square of its length.
Expert
The first moment of an RC tree’s impulse response: cheap, additive along a path and “faithful” (lowering it almost always lowers the true delay), so good for comparing options, but not accurate enough for signoff. It ignores the input edge shape, driver nonlinearity and inductance.
Explained in Clock tree synthesis (Design Flow).