Decontamination Chamber: Fortunately, these days, it just involves a light mist of nanites checking you out (while clothed, so no fanservice, alas), but it’s routine protocol for (physically) entering a new habitat or, indeed, planet, given the risks of disease and, worse, ecological contamination. It probably tells you everything
Alien Hair: Sometimes. Also, sometimes, fur, tentacles, symbiotic fungi, cilia, etc.
Alien Geometries: Something seen in automation-dominated infrastructure areas, where buildings and systems which are – for the most part – only ever used by AI tend to be made in complexes of simple geometric shapes, lines, and curves, without any of the softening or decoration that most sophont eyes find comfortable.
Alien Blood: Played straight, in many varieties. For example, eldrae blood is close to indigo in color, due to the not-hemoglobin it uses (borrowed from Elieran bluelife, which is indeed mostly blue; and it would lead to noticeably different skin tones if they weren’t all so damn pale anyway)
The Aesthetics of Technology: Oh, the Imperials play this one completely straight. After all, the Imperial motto is Order, Progress, Liberty, and it would never do to make all those neophilic people think that they weren’t getting their Progress, capital-p included. So not only should it be all future-y,
Action Bomb: Spider mines. Also AKVs (Autonomous Kill Vehicles) who can use themselves as a kinetic-kill weapon when they run out of ammunition. Of course, they back themselves up first, so it’s not like they’re blowing themselves up permanently.
Abusing the Kardashev Scale For Fun and Profit: Working their way up to this right now, the Empire has a couple of Dyson spheres, one under construction, the other completely dedicated to energy generation, plus other energy-generation facilities scattered here and there and about the place. This makes them, I
Absurdly Spacious Sewer: Not sewers, technically, but Imperial cities are known for their lengthy networks of service tunnels running under the streets, through which the actual pipes and cables that supply water, sewage & garbage removal, power, nanoslurry, bandwidth, and other utilities are routed, along with the pneumatic mail system
Abandon Ship: As mentioned under Escape Pod, only occasionally useful in the physical sense due to the lack of places the escape pods could take you, given their delta-v constraints – nor are they likely to have enough delta-V to correct a sunward plummet, or other highly inadvisable course – and the
Escape Pod: Averted. Fusion reactors don’t explode, they fizzle. Antimatter reactors (well, really, the storage cryocels) do explode, but in general, there’s no way you’re going to get away from them fast enough in order to matter. And unless you’re in orbit around a planet or
Animate Inanimate Object: Not to the full extent of this trope, but due to the impact of ubiquitous computing and the generally spime-ish and MEMS-ized nature of Stuff, it’s true enough to make the animists happy.
Location Theme Naming: Sometimes. There’s a class of battleships named after Imperial habitable planets, a class of oilers named after gas giants, a class of cruisers named after Elieran mountains, a class of frigates named after well-known drift-habitats…