The Collaborative International Dictionary of English v.0.48:

Baptism \Bap"tism\, n. [OE. baptim, baptem, OF. baptesme,
   batisme, F. bapt[^e]me, L. baptisma, fr. Gr. ba`ptisma, fr.
   bapti`zein to baptize, fr. ba`ptein to dip in water, akin to
   baqy`s deep, Skr. g[=a]h to dip, bathe, v. i.]
   The act of baptizing; the application of water to a person,
   as a sacrament or religious ceremony, by which he is
   initiated into the visible church of Christ. This is
   performed by immersion, sprinkling, or pouring.
   [1913 Webster]

WordNet (r) 3.0 (2006):

baptism
    n 1: a Christian sacrament signifying spiritual cleansing and
         rebirth; "most churches baptize infants but some insist on
         adult baptism"

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "baptism":
   acceptance, admission, admittance, affusion, appellation,
   aspergation, aspersion, baptismal gown, baptismal regeneration,
   baptistery, baptizement, bath, bathing, bedewing, burial, calling,
   chrismal, christening, confirmation, dampening, damping,
   definition, deluge, denomination, designation, dewing, dip,
   dipping, dousing, drowning, duck, ducking, dunking, engulfment,
   enlistment, enrollment, extreme unction, flooding, font,
   holy orders, hosing, hosing down, humidification, identification,
   immergence, immersion, immission, inauguration, induction,
   infusion, initiation, installation, instatement, intromission,
   inundation, investiture, irrigation, laving, matrimony, moistening,
   naming, nicknaming, ordination, penance, rinsing, seven sacraments,
   sinking, souse, sousing, sparging, spattering, splashing,
   splattering, spraying, sprinkling, styling, submergence,
   submersion, swashing, terming, the Eucharist, total immersion,
   watering, wetting



The Devil's Dictionary (1881-1906):

BAPTISM, n.  A sacred rite of such efficacy that he who finds himself
in heaven without having undergone it will be unhappy forever.  It is
performed with water in two ways -- by immersion, or plunging, and by
aspersion, or sprinkling.

    But whether the plan of immersion
    Is better than simple aspersion
        Let those immersed
        And those aspersed
    Decide by the Authorized Version,
    And by matching their agues tertian.
                                                                  G.J.