What is "compound radical"

Radical \Rad"i*cal\ (r[a^]d"[i^]*kal), n.

1. (Philol.)

   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.

      The words we at present make use of, and understand only by common agreement, assume a new air and life in the understanding, when you trace them to their radicals, where you find every word strongly stamped with nature; full of energy, meaning, character, painting, and poetry.
      --Cleland.

2. (Politics) One who advocates radical changes in government or social institutions, especially such changes as are intended to level class inequalities; -- opposed to conservative.

   In politics they [the Independents] were, to use the phrase of their own time, ``Root-and-Branch men,'' or, to use the kindred phrase of our own, Radicals.
   --Macaulay.

3. (Chem.)

   1. A characteristic, essential, and fundamental constituent of any compound; hence, sometimes, an atom.

      As a general rule, the metallic atoms are basic radicals, while the nonmetallic atoms are acid radicals.
      --J. P. Cooke.

   2. 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.

4. (Alg.) A radical quantity. See under Radical, a.

   An indicated root of a perfect power of the degree indicated is not a radical but a rational quantity under a radical form.
   --Davies & Peck (Math. Dict.)

5. (Anat.) A radical vessel. See under Radical, a.