The Collaborative International Dictionary of English v.0.48:

Intension \In*ten"sion\, n. [L. intensio: cf. F. intension. See
   Intend, and cf. Intention.]
   [1913 Webster]
   1. A straining, stretching, or bending; the state of being
      strained; as, the intension of a musical string.
      [1913 Webster]

   2. Increase of power or energy of any quality or thing;
      intenseness; fervency. --Jer. Taylor.
      [1913 Webster]

            Sounds . . . likewise do rise and fall with the
            intension or remission of the wind.   --Bacon.
      [1913 Webster]

   3. (Logic & Metaph.) The collective attributes, qualities, or
      marks that make up a complex general notion; the
      comprehension, content, or connotation; -- opposed to
      extension, extent, or sphere.
      [1913 Webster]

            This law is, that the intension of our knowledge is
            in the inverse ratio of its extension. --Sir W.
                                                  Hamilton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intension
    n 1: what you must know in order to determine the reference of
         an expression [syn: intension, connotation]