Freedom fighter, terrorist,revolutionary. Who cares? The label attached depends on the side of the political fence it is being viewed from.
The terms "terrorist" and "terrorism" originated during the French Revolution of the late 18th century but gained mainstream popularity during the U.S. presidency of Ronald. It was also applied to the activities of Irgun in the late 40's
Interestingly the Malayan Emergency
             the Mau Mau emergency
             the Cyprus emergency. I do not believe the word terrorist was used.
I would be quite happy to accept a definition of a terrorist as being an irregular army in opposition to the Internationally recognised government.

I really do not understand your problem. Mandela's convictions are there for all to see. The activities of his organisation clearly encompass acts that any rational person would accept as terrorism.
Trying to use semantics to argue a flawed man should be sanctified is plainly nonsense.
    Why can you not accept what he demonstrably was in his early life.

What he became, and represented in his later life, is another matter entirely.

I believe that Mandela alone made the transition in South Africa relatively painless and had a tremendous impact on the peaceful evolution of modern South Africa. Without him it would have been a blood bath. I was actually there when the major riots occurred and saw the subsequent chinks slowly open in the apartheid policy. How much of that subsequent change was caused by Mandela's party and how much was driven by the external changes in Mozambique, Angola and Rhodesia is a moot point, but without Mandela presiding over the changeover, the outcome would have been very different.