The Collaborative International Dictionary of English v.0.48:

Idyl \I"dyl\, n. [L. idyllium, Gr. ?, fr. ? form; literally, a
   little form of image: cf. F. idylle. See Idol.]
   A short poem; properly, a short pastoral poem; as, the idyls
   of Theocritus; also, any poem, especially a narrative or
   descriptive poem, written in an eleveted and highly finished
   style; also, by extension, any artless and easily flowing
   description, either in poetry or prose, of simple, rustic
   life, of pastoral scenes, and the like. [Written also
   idyll.]
   [1913 Webster]

         Wordsworth's solemn-thoughted idyl.      --Mrs.
                                                  Browning.
   [1913 Webster]

         His [Goldsmith's] lovely idyl of the Vicar's home. --F.
                                                  Harrison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

idyll
    n 1: an episode of such pastoral or romantic charm as to qualify
         as the subject of a poetic idyll
    2: a musical composition that evokes rural life [syn:
       pastorale, pastoral, idyll, idyl]
    3: a short poem descriptive of rural or pastoral life [syn:
       eclogue, bucolic, idyll, idyl]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "idyll":
   English sonnet, Horatian ode, Italian sonnet, Petrarchan sonnet,
   Pindaric ode, Sapphic ode, Shakespearean sonnet, alba, anacreontic,
   balada, ballad, ballade, bucolic, canso, chanson, clerihew, dirge,
   dithyramb, eclogue, elegy, epic, epigram, epithalamium, epode,
   epopee, epopoeia, epos, georgic, ghazel, haiku, jingle, limerick,
   lyric, madrigal, monody, narrative poem, nursery rhyme, ode,
   palinode, pastoral, pastoral elegy, pastorela, pastourelle, poem,
   prothalamium, rhyme, rondeau, rondel, roundel, roundelay, satire,
   sestina, sloka, song, sonnet, sonnet sequence, tanka, tenso,
   tenzone, threnody, triolet, troubadour poem, verse, verselet,
   versicle, villanelle, virelay