Source : Webster's Revised Unabridged Dictionary (1913)

Insure \In"sure\, v. t. [imp. & p. p. {Insured}; p. pr. & vb. n.
   {Insuring}.] [OE. ensuren, prob. for assuren, by a change of
   prefix. See 1st {In-}, and {Sure}, and cf. {Assure},
   {Ensure}.] [Written also {ensure}.]
   1. To make sure or secure; as, to insure safety to any one.

   2. Specifically, to secure against a loss by a contingent
      event, on certain stipulated conditions, or at a given
      rate or premium; to give or to take an insurance on or
      for; as, a merchant insures his ship or its cargo, or
      both, against the dangers of the sea; goods and buildings
      are insured against fire or water; persons are insured
      against sickness, accident, or death; and sometimes
      hazardous debts are insured.

Insure \In*sure"\, v. i.
   To underwrite; to make insurance; as, a company insures at
   three per cent.

Source : WordNet®

insure
     v 1: be careful or certain to do something; make certain of
          something; "He verified that the valves were closed";
          "See that the curtains are closed"; "control the quality
          of the product" [syn: {see}, {check}, {see to it}, {ensure},
           {control}, {ascertain}, {assure}]
     2: make certain of; "This nest egg will ensure a nice
        retirement for us"; "Preparation will guarantee success!"
        [syn: {guarantee}, {ensure}, {assure}, {secure}]
     3: protect by insurance; "The insurance won't cover this" [syn:
         {cover}, {underwrite}]
     4: take out insurance for