What's a “delay line”?

I know what a delay is, but what's a delay line?
What's the reason for calling it a "delay line"?

• My guess is that it relates to the historical use of electrical or acoustic propagation time down a physical object, such as a wire "line" or pipe "line", for time delaying signals by a deterministic and known amount. – hotpaw2 Dec 23 '15 at 19:47
• I wonder if using the term "delay line" is a bit of a misnormer in digital implementations. en.wikipedia.org/wiki/Digital_delay_line, but is there a line in the digital implementation? – mavavilj Dec 23 '15 at 20:02
• $z^{-N}$ ...... – robert bristow-johnson Dec 24 '15 at 6:18
• hotpaw2: I agree. I once worked a commercial radio station and they had a rack-mounted device containing a stainless steel spring about 5" in length. They would apply a rotational torque to one end of the spring at audio frequencies. The vibration wave traveled down the spring and was sensed by a piezoelectric sensor at the far end. Adding the sensor output voltage to the original audio signal produced a desirable echo. They called that device "the delay line." – Richard Lyons Dec 25 '15 at 11:53

In the discrete (digital) domain, I've more usually heard delay lines called tapped delay lines. That's because each sample is a delay, and to form an FIR filter, we need access (to tap into) the signal at different delays. The line of $z^{-1}$ boxes in the diagram below is what I would call a tapped delay line.