So if someone blows up a transformer substation, but no one dies, is that terrorism?

What about a building? A government office? An empty school?

Or here's a better question: what then do you call the use of criminal means, for political purposes, to gain compliance from local population and political authorities, when it doesn't include direct harm against humans and focuses on destruction of property?

Terrorism is acts to instil fear for political ends. Intent matters. If someone blows up a building to make people fearful, then it's terrorism. If someone does it to destroy someone's property, then its vandalism. By your logic, striking and blocking scabs is terrorism.

And so in the current moment, why do you think the Houthis (if they are indeed responsible, but let's assume so for the sake of this argument) are cutting submarine cables?

What do you think the intent behind that was?

I would guess, to be as much of a nuisance as possible to those they percieve themselves to be at war with with what limited capabilities they have. They can't possibly believe that this will have significant economic impact so as to hamper their enemies' capability to continue to wage war. This will strike fear into the hearts of no one. But it's damage, and they can do it, so they do.

So it's the use of destructive measures, which one might describe as "violence", targeted at assets utilized principally by civilians, for the purposes of affecting political change in policy by a government.

Were this to rise in severity from "nuisance", what would you describe that feeling as? What reaction do you think commonly provokes people exposed to negative stimuli into action they don't otherwise want to take?

No. Man, quit trying to massage the definition to make yourself right.

That's dodging the question not a rebuttal.

I don't need to rebut you, you're stretching the definition of "terrorism" to feel right. There's no question to dodge, "so what happens if this activity escalates into terrorism, would that make this terrorism?" The discussion was settled 2 comments ago. "Terrorism" is clearly defined. Breaking things to inconvenience your enemies in warfare is not terrorism.

Terrorism is the use of violence and intimidation, especially against civilians, for political purposes. [merriam Webster]

Taking out a communication cable is not that by any stretch of your quite substantial definitions below.