Nowhere in Trek is it said that warp means faster than light.
From Voyager:
Janeway: "Tom. What's the first thing they teach you about Warp?"
Paris: "Uh... faster than light, no left or right... when possible, maintain a linear trajectory".
I'd say it's pretty well established that Warp = faster than light, going so far as to even stating on numerous occasions just how fast Warp factors are (depending on the show and script/drama needs of course - all of which were much higher than the 300 000 km/s - which is light speed).
Also, First Contact, moments before the Phoenix jumped into warp and was shaking a lot, Riker yelled: 'Approaching light speed.' - followed by Cochrane: "We're at critical velocity".
This one I'll grant you 'COULD' be interpreted as Warp speed equaling light speed, but I find it doubtful.
Only in Enterprise did we have an example of when the dialogue completely contradicted Warp = faster than light and it seemed as if the ship was moving at several times SLOWER than the speed of light, regardless of the show's dialogue also stating on several occasions that Warp = faster than light - shining example of the show contradicting itself and the writers messing it up - there's no escaping that.
Unless you want to state that Warp speed suddenly became slower than light speed (in which case it would take the ship a LONG time to get ANYWHERE in the known area of space).
Nor is it established that warp 1 equals lightspeed, for that matter. If the backstage ideas on this technobabble don't please you, feel free to ignore them and substitute your own.
Canon-wise (except for the dialogue in Enterprise royally messing things up and contradicting itself several times in the same episode), nothing really contradicts the well repeated premise that Warp is faster than light (at least not in shows other than Enterprise).
And yes, on-screen wise, virtually nothing indicates that Warp1 EQUALS light speed (however the mere meaning of Warp speed is kept repeated through Trek as being FASTER than light - actual Warp factors are usually placed into that category automatically).
As for the definition of "sustainable" as "sustainable till infinity", it just plain isn't real. if a dictionary uses that definition, then the dictionary is wrong. Real usage always trumps dictionaries.
Sustainable as a term was used by Janeway to pose a question to Paris right after they took Voyager back in 'Basics Part 2'.
Janeway: "Mr. Paris! Is this ship capable of sustained flight?".
Paris: "All of propulsion and navigational systems are online captain".
Janeway: "Good. Take us away from our new home, and set a course for the old one Mr. Paris. Warp 8!"
Sustained flight in this context would probably mean 'for as long until I say otherwise/or we have the resources'.
Also, Voyager was repeatedly stated to have a 'top cruising speed' of Warp 9.975
Cruising speed in contemporary human terminology means: 'where flight is most fuel efficient' (taken from Wiki).
So if we would to apply this to Voyager, then the ship should be able to sustain Warp 9.975 for as long as it has the power/resources to do so.
I'm not arguing the writers of VOY didn't screw up with their references to the sustainable speed of the hero ship. I'm simply pointing out that in this context, the usage of "sustainable" would be perfectly correct if it referred to hours rather than centuries of sustainability.
Timo Saloniemi
Except that in case of Star Trek, 'sustainable' was never defined as meaning 'for short periods of time - such as minutes or hours', unless of course it was explicitly stated as such in order to increase the drama.
For example, Paris stated that Voyager wouldn't be able to sustain the quantum stresses of QS drive for more than 1 hour.
In that particular scenario, 'sustainable' was limited in duration, hence your version applies, but most other times it was not.
Besides, Voyager didn't use the phrase 'sustainable', instead, it used the term 'top cruising speed'.
To me it always meant that the ship would be able to hold Warp 9.975 for as long as it had the fuel/resources to do so.