After grepping through official documentation, it seems that “Vim script” is the “most” official name since nowhere is “VimScript” mentioned in regular sentences. GetLatestVimScripts is often mentioned, but this is a script name where no underlines are allowed.
I’d guess VimL is just an abbreviation for Vim Language (Vim script). It was probably coined some time ago and became used at few places, but it’s not official.