The Collaborative International Dictionary of English v.0.48:

Rood \Rood\ (r[=oo]d), n. [AS. r[=o]d a cross; akin to OS.
   r[=o]da, D. roede rod, G. ruthe, rute, OHG. ruota. Cf. Rod
   a measure.]
   1. A representation in sculpture or in painting of the cross
      with Christ hanging on it.
      [1913 Webster]

   Note: Generally, the Trinity is represented, the Father as an
         elderly man fully clothed, with a nimbus around his
         head, and holding the cross on which the Son is
         represented as crucified, the Holy Spirit descending in
         the form of a dove near the Son's head. Figures of the
         Virgin Mary and of St. John are often placed near the
         principal figures.
         [1913 Webster]

               Savior, in thine image seen
               Bleeding on that precious rood.    --Wordsworth.
         [1913 Webster]

   2. A measure of five and a half yards in length; a rod; a
      perch; a pole. [Prov. Eng.]
      [1913 Webster]

   3. The fourth part of an acre, or forty square rods.
      [1913 Webster]

   By the rood, by the cross; -- a phrase formerly used in
      swearing. "No, by the rood, not so." --Shak.

   Rood beam (Arch.), a beam across the chancel of a church,
      supporting the rood.

   Rood loft (Arch.), a loft or gallery, in a church, on which
      the rood and its appendages were set up to view. --Gwilt.

   Rood screen (Arch.), a screen, between the choir and the
      body of the church, over which the rood was placed.
      --Fairholt.

   Rood tower (Arch.), a tower at the intersection of the nave
      and transept of a church; -- when crowned with a spire it
      was called also rood steeple. --Weale.

   Rood tree, the cross. [Obs.] "Died upon the rood tree."
      --Gower.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rood
    n 1: representation of the cross on which Jesus died [syn:
         crucifix, rood, rood-tree]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "rood":
   Agnus Dei, Calvary cross, Christogram, Greek cross, Holy Grail,
   Host, Jerusalem cross, Latin cross, Maltese cross, Pieta,
   Russian cross, Sanctus bell, Sangraal, T, X, ankh, ark, asperger,
   asperges, aspergillum, avellan cross, bambino, beadroll, beads,
   candle, censer, chaplet, chi, chi-rho, christcross, ciborium,
   crisscross, cross, cross ancre, cross botonee, cross bourdonee,
   cross fitche, cross fleury, cross formee, cross fourchee,
   cross grignolee, cross moline, cross of Cleves, cross of Lorraine,
   cross patee, cross recercelee, cross-crosslet, crossbones,
   crosslet, crucifix, cruciform, cruet, crux, crux ansata,
   crux capitata, crux decussata, crux gammata, crux immissa,
   crux ordinaria, dagger, eucharistial, ex, exing, fork cross,
   gammadion, holy cross, holy water, holy-water sprinkler, icon,
   incensory, inverted cross, long cross, matzo, menorah, mezuzah,
   mikvah, monstrance, osculatory, ostensorium, papal cross,
   paschal candle, pax, pectoral cross, phylacteries, potent cross,
   prayer shawl, prayer wheel, pyx, relics, rosary, sacramental,
   sacred relics, sacring bell, saltire, shofar, sukkah, swastika,
   tabernacle, tallith, tau, thurible, trefled cross, urceole,
   veronica, vigil light, voided cross, votive candle