Alternate timeline and alternate universe are the same thing.
Nero created the alternate timeline by simply appearing in the past and changing things. It's alternate because it's not what happened originally. Up until the "lightning storm in space" which diverted the Kelvin from whatever they were doing, everything was on track for TOS, TNG etc to "happen".
Is it alternate as well as
or alternate instead of
? That's open to debate.
The many-worlds theory (the current real-life accepted time travel theory) says a timeline can't be destroyed. Thus TOS is still "out there" somewhen, akin to the mirror universe (which diverged from ours long ago, judging by the "In a Mirror, Darkly" intro) or the many slightly different ones in "Parallels" (one of which diverged as recently as "Best of Both Worlds").
This does make a mess of "Endgame", where Old Janeway attempts to "fix" the past and bring her crew home intact, as well as other episodes and movies. By MW, the future she left is unchanged.