What a space ship is.... what Serenity really is... is freedom.
It's the wide open sky, with the stars lighting your way across the Verse in all their brilliant glory.
It's all of that vastness stretching front of you and trailing behind you, surround you in all directions while you have to do is pick a planet and aim for it.
It's going where ever you choose, when you damn well wish to, without being beholding to anyone except yourself, your crew and your cargo.
It's standing on the bridge with Zoe and Wash.
What Serenity really is... is happiness.