No, reboot implies that something has been redone but differently, like nuBSG.
In recent fan vernacular, yes, but only because BSG used it that way. In industry-speak, a reboot is any restart/reinvigoration of a dormant franchise that makes it viable and popular again, regardless of whether it's a new continuity like BSG or just a fresh take on an old continuity like
Doctor Who (or Abrams' ST, which is somewhere between the two). It's like rebooting your computer -- that doesn't mean changing it into a different computer, it just means taking something that's inactive and giving it a fresh start. But because BSG is a recent, prominent example of a reboot, fans have jumped to the conclusion that its approach is the
definition of a reboot, which really doesn't follow.
I believe the term "relaunch" was initially meant to refer only to the process of setting the post-finale DS9 novel line in motion, the promotional push that started it out. After all, if you refit a ship and launch it again, then the term "relaunch" refers to the
act of sending out the new ship; you don't call the ship itself a relaunch. So the term was never meant to apply to the actual book line as a whole, just to the marketing strategy of starting it out with a big publicity push and a rapid-fire release schedule for the first few installments.
But people on the Internet have a knack for repurposing vocabulary, especially, it seems, when it begins with "re-" (such as "reboot" or "remastered"). And so the term "relaunch" started being applied to the post-finale DS9 series as a whole, and then when other post-finale series came along, people started calling them relaunches as well.