The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]
   1. To make sure or secure; as, to insure safety to any one.
      [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ensure \En*sure"\, v. t.
   1. To make sure. See Insure.
      [1913 Webster]

   2. To betroth. [Obs.] --Sir T. More.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ensure
    v 1: make certain of; "This nest egg will ensure a nice
         retirement for us"; "Preparation will guarantee success!"
         [syn: guarantee, ensure, insure, assure, secure]
    2: 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, insure, see to it,
       ensure, control, ascertain, assure]