Presumably transparent aluminum. Probably a newer heavy transparent metal of some type. However, when the Ent-D saucer crashed in Generations, the windows seem to smash like ordinary glass. So maybe it is some type of glass composite that uses a structural integrity field. When the field is gone it becomes weaker.
It has to be either transparent aluminum or something better than that. I say that because, if you have transparent aluminum, the only reason not to use it is if you have something better. Either way, it should not shatter.
In First Contact Picard shows Lily the view of earth. She is surprised that there is no glass and Picard shows her that it is a forcefield.
Well, guys are always trying to impress women with their best toys. You didn't think he would take her to the plain old transparent aluminum window, did you?
Glass in the 24th century is just like glass here twenty years or thirty years ago. Except when it's not. It can hold whales and a lot of water. Or it can shatter like the dreams of a teenage girl in Harvey Weinstein's office when you whack with a phaser rifle. It can withstand the vacuum of space. Or it can fall over and shatter like the hopes of more followers in the Flat Earth Society when it falls on Beverly. It can withstand the crushing depths of a planet made of only water. Or with some shake, rattle and rolls of cracks open above your hear.
That scene is all kinds of weird though. That looked more like some kind of hatch than a window, as it had a huge metal door that had to be opened through a control console. A tiny room with a single console, that you could only get to through a tube, with a hatch in it. Weird, weird scene. We've also seen people in the reflections of windows many times.
It's regular glass, but with magic invisible fields running through it, making it as strong as the ship's hull (see: very end of Star Trek, where the window cracks at the same rate as the walls). Thus, when the ship crashes and dies (Generations, Into Darkness and Beyond), the glass becomes as breakable as the windows in your home.
Found the line in question. DATA: I detect no unusual readings along standard parameters. Curious. The transparent aluminum alloy of this window is exhibiting a pattern of transient electrical currents. There you have it, folks; end of thread. As for glass shattering in the scenes mentioned in this thread, we can rationalize as those scenes taking place in alternate universes. I've seen that rationale pop up around here from time to time .
Those forcefields must require a lot of energy. Just wait for the warp engines and impulse engines to fail under attack, leaving only "the batteries". Forcefields will be down in no time. Then the crew will be up (and out) in no time. Previously jettisoned? Sufficiently cold temperatures, like how a scientist dips a lock into a vat of liquid nitrogen then whips out a hammer? Skip to 4:40 if you don't want to see food shamelessly wasted. Or "Balance of Terror" that showed Romulan weaponry making the toughest known alloy as brittle as a saltine cracker? Maybe transparent aluminum is fitted into windows, with other alloys for the rest of the hull and this is the first time we see windows shattered post-attack... then again, I don't recall Klingon disruptors hitting all 2000 windows, or the dome atop the bridge that does shatter... nope. It's just an effect meant for tension and drama.
Oh, so the Romulans just invented a huge can of freeze spray. Those silly Romulans. No wonder Kirk sent them to their grave.
One time when I was in the Army me and this lieutenant I worked with were outside pouring liquid nitrogen into a smaller container! It was so cool when it accidentally hit the lawn- frozen instantly, then dead. Then the Sergent major came up and yelled at us. I think maybe what we were doing wasn't considered safe.