The Collaborative International Dictionary of English v.0.48:

Serenade \Ser`e*nade"\, n. [F. s['e]r['e]nade, It. serenata,
   probably fr. L. serenus serene (cf. Serene), misunderstood
   as a derivative fr. L. serus late. Cf. Soir['e]e.] (Mus.)
   (a) Music sung or performed in the open air at nights; --
       usually applied to musical entertainments given in the
       open air at night, especially by gentlemen, in a spirit
       of gallantry, under the windows of ladies.
   (b) A piece of music suitable to be performed at such times.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Serenade \Ser`e*nade"\, v. t. [imp. & p. p. Serenaded; p. pr.
   & vb. n. Serenading.]
   To entertain with a serenade.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Serenade \Ser`e*nade"\, v. i.
   To perform a serenade.
   [1913 Webster]

WordNet (r) 3.0 (2006):

serenade
    n 1: a musical composition in several movements; has no fixed
         form [syn: divertimento, serenade]
    2: a song characteristically played outside the house of a woman
    v 1: sing and play for somebody; "She was serenaded by her
         admirers"

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "serenade":
   Brautlied, Christmas carol, Kunstlied, Liebeslied, Volkslied,
   address, addresses, alba, amorous pursuit, anthem, art song,
   aubade, ballad, ballade, ballata, barcarole, beau, blues,
   blues song, boat song, bridal hymn, brindisi, calypso, canso,
   canticle, canzone, canzonet, canzonetta, carol, cavatina, chanson,
   chant, chantey, chase, chirp, chirrup, choir, chorus, court,
   courting, croon, croon song, descant, dirge, ditty, do-re-mi,
   drinking song, epithalamium, esquire, folk song, follow, gallantry,
   hum, hymeneal, hymn, intonate, intone, lay, lay siege to, lied,
   lilt, love song, love-lilt, make suit to, matin, minstrel,
   minstrel song, minstrelsy, national anthem, pay attention to,
   pay court to, pipe, prothalamium, psalm, pursue, quaver, roulade,
   serena, serenata, shake, sing, sing in chorus, sol-fa, solmizate,
   song, spark, squire, sue, suing, suit, swain, sweetheart,
   theme song, torch song, tremolo, trill, troll, tweedle, tweedledee,
   twit, twitter, vocalize, war song, warble, wedding song, whistle,
   woo, wooing, yodel