Apparently Elon has a thing for the letter X. SpaceX, naming his son "X Æ A-12", and now renaming Twitter.

X.com was the startup Musk (and others) launched in 1999. It merged with another startup to create PayPal in 2000. Musk bought the X.com domain back a few years ago.

But SpaceX at least is only short for SpaceeXploration

But they don’t do exploration? They just do orbit stuff