1 basic, fundamental, elementary, inherent, constitutional, elemental, essential, cardinal, principal, primary, deep, deep-seated, profound, underlying, organic, natural, rudimentary:
There is a radical error in your approach to the problem. There are radical differences in the ways we regard the English language
2 thorough, thoroughgoing, complete, entire, total, exhaustive, sweeping, (all-)inclusive, comprehensive, all-embracing, out-and-out, drastic, severe, extreme, extremist, revolutionary:
We have made radical changes in the way we live. Every political party that is not in power promises radical reform
3 extremist, revolutionary, fanatic(al), militant, anarchist(ic), immoderate:
A staunch conservative, Vincent regarded as radical anyone who disagreed with him.
4 extremist, revolutionary, fanatic, zealot, immoderate, anarchist, militant:
The radicals favour reform, the conservatives the status quo - but there is nothing new in that.
5 communist, leftist, left-winger, red, Bolshevik, Bolshevist, pink, Slang US pinko:
Carlo, recently arrested as a spy, had been a radical since his days at university.