basic, essential, fundamental, radical, vital

名詞 radical:

1. (chemistry) two or more atoms bound together as a single unit and forming part of a molecule
   同義詞：group, chemical group
2. an atom or group of atoms with at least one unpaired electron; in the body it is usually an oxygen molecule than has lost an electron and will stabilize itself by stealing an electron from a nearby molecule
   同義詞：free radical
3. a person who has radical ideas or opinions
4. a character conveying the lexical meaning of a logogram
5. a sign placed in front of an expression to denote that a root is to be extracted
   同義詞：radical sign
6. (linguistics) the form of a word after all affixes are removed
   同義詞：root, root word, base, stem, theme

形容詞 radical:

1. (used of opinions and actions) far beyond the norm
   同義詞：extremist, ultra
2. markedly new or introducing radical change
   同義詞：revolutionary
3. arising from or going to the root
4. of or relating to or constituting a linguistic root
5. especially of leaves; located at the base of a plant or stem; especially arising directly from the root or rootstock or a root-like stem
   同義詞：basal

1. I don't belong to the school of thought that favours radical change.
   我不屬於激進派。
2. Radical political views.
   激進的政治觀點
3. Ted is a fire-eating radical.
   泰德是個咄咄逼人的急進份子。
4. They called for a radical reform of our tax system.
   他們要求對稅收制度進行根本改革。
5. The talks are aimed at radical reductions in the level of weapons.
   那些會談都以大幅度裁減軍備為目標。

rad.i.cal

adj.（形容詞）
Arising from or going to a root or source; basic:
根本的，根源的：產(chǎn)生于或到達根部或源泉的；基礎的：
例句：
a radical flaw in a plan; chose the radical solution of starting all over again.
計劃的根本性缺陷；選擇重新開始的根本解決方法

Departing markedly from the usual or customary; extreme:
極端的，徹底的：明顯背離常規(guī)或習慣的；極端的：
例句：
radical opinions on education.
對教育的極端的見解

Favoring or effecting fundamental or revolutionary changes in current practices, conditions, or institutions:
激進的，改革的：支持或引起目前的慣例、條件或制度根本性或革命性的變化的：
例句：
radical political views.
激進的政治觀點

Linguistics Of or being a root:
【語言學】 詞根的：屬于或作為詞根的：
例句：
a radical form.
詞根形式

Botany Arising from the root or its crown:
【植物學】 根生的，根的：生于根部或冠部的：
例句：
radical leaves.
根生的葉

n.（名詞）
One who advocates fundamental or revolutionary changes in current practices, conditions, or institutions:
激進分子：在目前的習慣、條件或制度下鼓動根本性或革命性改變的人：
例句：
radicals seeking to overthrow the social order.
企圖擾亂社會次序的激進分子

Abbr. rad.Mathematics The root of a quantity as indicated by the radical sign.
縮寫 rad.【數(shù)學】 根，根號：用根號表示出來的一個數(shù)的根
Symbol R An atom or a group of atoms with at least one unpaired electron.
符號 R 原子因，基團：至少有一個游離電子的原子或一組原子
Abbr. rad.Linguistics See root
縮寫 rad.【語言學】 參見 root

來源：
Middle English [of a root]
中古英語 [根]
from Late Latin r3doc3lis [having roots]
源自 后期拉丁語 r3doc3lis [有根的]
from Latin r3dox r3doc- [root] * see wr3d-
源自 拉丁語 r3dox r3doc- [根] *參見 wr3d-

【引伸】
rad.ically
adv.（副詞）
rad.icalness
n.（名詞）