radical

a.

  1. Of or pertaining to the root; proceeding directly from the root.
  2. Hence: Of or pertaining to the root or origin; reaching to the center, to the foundation, to the ultimate sources, to the principles, or the like; original; fundamental; thorough-going; unsparing; extreme; as, radical evils; radical reform; a radical party.
  3. Belonging to, or proceeding from, the root of a plant; as, radical tubers or hairs.
  4. Proceeding from a rootlike stem, or one which does not rise above the ground; as, the radical leaves of the dandelion and the sidesaddle flower.
  5. Relating, or belonging, to the root, or ultimate source of derivation; as, a radical verbal form.
  6. Of or pertaining to a radix or root; as, a radical quantity; a radical sign. See below.
  7. A radical vessel. See under Radical, a.

n.

  1. A primitive word; a radix, root, or simple, underived, uncompounded word; an etymon.
  2. A primitive letter; a letter that belongs to the radix.
  3. One who advocates radical changes in government or social institutions, especially such changes as are intended to level class inequalities; -- opposed to conservative.
  4. A characteristic, essential, and fundamental constituent of any compound; hence, sometimes, an atom.
  5. Specifically, a group of two or more atoms, not completely saturated, which are so linked that their union implies certain properties, and are conveniently regarded as playing the part of a single atom; a residue; -- called also a compound radical. Cf. Residue.
  6. A radical quantity. See under Radical, a.