A vascular malformation resulting from a developmental error of venous tissue composed of dysmorphic channels lined by flattened endothelium and exhibiting slow turnover. A venous malformation may present as a blue patch on the skin ranging to a soft blue mass. Venous malformations are easily compressible and usually swell in thewhen venous pressure increases (e.g., when held in a dependent position or when a child cries). They may be relatively localized or quite extensive within an anatomic region. The phenotype is Venous malformation (Human Phenotype Ontology id HP_0012721). Also known as: Venous malformations.