Source : Webster's Revised Unabridged Dictionary (1913)

   2. Participation in advantage, profit, and responsibility;
      share; portion; part; as, an interest in a brewery; he has
      parted with his interest in the stocks.

   3. Advantage, personal or general; good, regarded as a
      selfish benefit; profit; benefit.

            Divisions hinder the common interest and public
            good.                                 --Sir W.
                                                  Temple.

            When interest calls of all her sneaking train.
                                                  --Pope.

   4. Premium paid for the use of money, -- usually reckoned as
      a percentage; as, interest at five per cent per annum on
      ten thousand dollars.

            They have told their money, and let out Their coin
            upon large interest.                  --Shak.

   5. Any excess of advantage over and above an exact equivalent
      for what is given or rendered.

            You shall have your desires with interest. --Shak.

   6. The persons interested in any particular business or
      measure, taken collectively; as, the iron interest; the
      cotton interest.

   {Compound interest}, interest, not only on the original
      principal, but also on unpaid interest from the time it
      fell due.

   {Simple interest}, interest on the principal sum without
      interest on overdue interest.



   12. (Min.) Homogenous.

   13. (Zo["o]l.) Consisting of a single individual or zooid;
       as, a simple ascidian; -- opposed to compound.

   {Simple contract} (Law), any contract, whether verbal or
      written, which is not of record or under seal. --J. W.
      Smith. --Chitty.

   {Simple equation} (Alg.), an eqyation containing but one
      unknown quantity, and that quantity only in the first
      degree.

   {Simple eye} (Zo["o]l.), an eye having a single lens; --
      opposed to {compound eye}.

   {Simple interest}. See under {Interest}.

   {Simple larceny}. (Law) See under {Larceny}.

   {Simple obligation} (Rom. Law), an obligation which does not
      depend for its execution upon any event provided for by
      the parties, or is not to become void on the happening of
      any such event. --Burrill.

   Syn: Single; uncompounded; unmingled; unmixed; mere;
        uncombined; elementary; plain; artless; sincere;
        harmless; undesigning; frank; open; unaffected;
        inartificial; unadorned; credulous; silly; foolish;
        shallow; unwise.

   Usage: {Simple}, {Silly}. One who is simple is sincere,
          unaffected, and inexperienced in duplicity, -- hence
          liable to be duped. A silly person is one who is
          ignorant or weak and also self-confident; hence, one
          who shows in speech and act a lack of good sense.
          Simplicity is incompatible with duplicity, artfulness,
          or vanity, while silliness is consistent with all
          three. Simplicity denotes lack of knowledge or of
          guile; silliness denotes want of judgment or right
          purpose, a defect of character as well as of
          education.

                I am a simple woman, much too weak To oppose
                your cunning.                     --Shak.

                He is the companion of the silliest people in
                their most silly pleasure; he is ready for every
                impertinent entertainment and diversion. --Law.

Source : WordNet®

simple interest
     n : interest paid on the principal alone