Source : Webster's Revised Unabridged Dictionary (1913)

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.

            So they do nothing, 't is a venial slip. --Shak.

   2. Allowed; permitted. [Obs.] ``Permitting him the while
      venial discourse unblamed.'' --Milton.

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

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.

            Whosoever committeth sin is the servant of sin.
                                                  --John viii.
                                                  34.

            Sin is the transgression of the law.  --1 John iii.
                                                  4.

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

            Enthralled By sin to foul, exorbitant desires.
                                                  --Milton.

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

            I grant that poetry's a crying sin.   --Pope.

   3. A sin offering; a sacrifice for sin.

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

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

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

   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.

   {Actual sin},

   {Canonical sins},

   {Original sin},

   {Venial sin}. See under {Actual}, {Canonical}, etc.

   {Deadly}, 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.

   Syn: Iniquity; wickedness; wrong. See {Crime}.

Source : WordNet®

venial sin
     n : a pardonable sin regarded as entailing only a partial loss
         of grace [ant: {mortal sin}]