To the extent that "terrorist" has any meaning, it is when a less powerful actor attempts to advance an agenda through creating an unmanageable situation for a more powerful institution by causing an increased sense of risk within individuals who are part of the support structure of that institution, thereby making it too expensive for the institution to continue preventing that agenda from being advanced.
Where conventional war wins by killing enough people on the other side that they can't stop you from doing what you want, instead terrorism makes people so expensive (in lost government support from a targeted public, increased salaries and heightened security, and/or lost inflows of money from targeted clients and customers) that allowing the terrorist agenda to advance turns out to be cheaper for the institution being attacked than continuing to fight.
That's what the ANC did.