Undocumented is a better term for the MOS 6502 in my opinion, because these opcodes aren't invalid, they can't be trapped and they don't throw an exception.
The 6502 was reimplemented in fresh silicon several times by different manufacturers, its more "undefined" than "undocumented". Some clones use them as actual new opcodes specific to that manufacturer. Some might do what the MOS chip did. Some might throw the processor into an unrecoverable state.
This is why I'm emphasizing MOS 6502. I'm well-familiar with the most popular alternative, WDC's 65C02, where all of the undocumented opcodes execute as NOP with no changes to internal registers/flags or to memory, but with one or two harmless side effects on timing behavior.
Seriously, stop using that word for things that aren't actually illegal.