There isn't a written definition, but it's exactly what common sense would dictate it to be. Ignore abilities do not stop it from entering battle, they do not target it in battle, and they do not protect the ignoring character from it.
Your opponent was correct that RTC can still Ignore Brown Enhancements even if their played on a Character who cannot be Ignored. However, TGT specifies that it Ignores Characters meeting certain requirements, not the entire Brigade, so a TGT Character has no shield against a Persian with Gallows up.