The Collaborative International Dictionary of English v.0.48:

Sin \Sin\, n. [OE. sinne, AS. synn, syn; akin to D. zonde, OS.
   sundia, OHG. sunta, G. s["u]nde, Icel., Dan. & Sw. synd, L.
   sons, sontis, guilty, perhaps originally from the p. pr. of
   the verb signifying, to be, and meaning, the one who it is.
   Cf. Authentic, Sooth.]
   1. Transgression of the law of God; disobedience of the
      divine command; any violation of God's will, either in
      purpose or conduct; moral deficiency in the character;
      iniquity; as, sins of omission and sins of commission.
      [1913 Webster]

            Whosoever committeth sin is the servant of sin.
                                                  --John viii.
                                                  34.
      [1913 Webster]

            Sin is the transgression of the law.  --1 John iii.
                                                  4.
      [1913 Webster]

            I think 't no sin.
            To cozen him that would unjustly win. --Shak.
      [1913 Webster]

            Enthralled
            By sin to foul, exorbitant desires.   --Milton.
      [1913 Webster]

   2. An offense, in general; a violation of propriety; a
      misdemeanor; as, a sin against good manners.
      [1913 Webster]

            I grant that poetry's a crying sin.   --Pope.
      [1913 Webster]

   3. A sin offering; a sacrifice for sin.
      [1913 Webster]

            He hath made him to be sin for us, who knew no sin.
                                                  --2 Cor. v.
                                                  21.
      [1913 Webster]

   4. An embodiment of sin; a very wicked person. [R.]
      [1913 Webster]

            Thy ambition,
            Thou scarlet sin, robbed this bewailing land
            Of noble Buckingham.                  --Shak.
      [1913 Webster]

   Note: Sin is used in the formation of some compound words of
         obvious signification; as, sin-born; sin-bred,
         sin-oppressed, sin-polluted, and the like.
         [1913 Webster]

   Actual sin, Canonical sins, Original sin, Venial sin.
      See under Actual, Canonical, etc.

   Deadly sins, or Mortal sins (R. C. Ch.), willful and
      deliberate transgressions, which take away divine grace;
      -- in distinction from vental sins. The seven deadly sins
      are pride, covetousness, lust, wrath, gluttony, envy, and
      sloth.

   Sin eater, a man who (according to a former practice in
      England) for a small gratuity ate a piece of bread laid on
      the chest of a dead person, whereby he was supposed to
      have taken the sins of the dead person upon himself.

   Sin offering, a sacrifice for sin; something offered as an
      expiation for sin.
      [1913 Webster]

   Syn: Iniquity; wickedness; wrong. See Crime.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Venial \Ve"ni*al\, a. [OF. venial, F. v['e]niel, L. venialis,
   from venia forgiveness, pardon, grace, favor, kindness; akin
   to venerari to venerate. See Venerate.]
   1. Capable of being forgiven; not heinous; excusable;
      pardonable; as, a venial fault or transgression.
      [1913 Webster]

            So they do nothing, 't is a venial slip. --Shak.
      [1913 Webster]

   2. Allowed; permitted. [Obs.] "Permitting him the while
      venial discourse unblamed." --Milton.
      [1913 Webster]

   Venial sin (R. C. Theol.), a sin which weakens, but does
      not wholly destroy, sanctifying grace, as do mortal, or
      deadly, sins.
      [1913 Webster] -- Ve"ni*al*ly, adv. -- Ve"ni*al*ness,
      n. --Bp. Hall.
      [1913 Webster]

WordNet (r) 3.0 (2006):

venial sin
    n 1: a pardonable sin regarded as entailing only a partial loss
         of grace [ant: deadly sin, mortal sin]