Cystic hygroma is
CYSTIC HYGROMA (Cavernous Lymphangioma)

It is a cystic swelling due to sequestration of a portion of jugular lymph sac from the lymphatic system, during the developmental period in utero.
Present at birth and so may cause obstructed labour. Occasionally present in early infancy.
Lymphatics fail to communicate to venous system; abnormal budding of lymphatics is seen.  
Often it is associated with Turner’s, Down’s, Klinefelter syndromes and trisomy 18 and 13.
It is also called as hydrocoele of the neck.
Lymphatic malformation (LM) type can be microcystic (lymphangioma) or macrocystic (cystic hygroma). It can cause lymphoedema, soft tissue and bony hypertrophy, asymmetry (face), macrochelia, macroglossia, macrotia, cellulitis.

​Key Concept:
 It is a type of congenital lymphangioma and the common victims are infants and children. 
Reference: SRB's Manual of Surgery, 5th ed, page no 86, 216, 435
A form of hydrocele
A branchial cleft cyst
Salivary gland enlargement
Lymphangioma