A developmental defect resulting in the presence of a thyroglossal cyst, a fibrous cyst that forms from a persistent thyroglossal duct. During the embryologic development of the thyroid gland, the precursor of the thyroid migrates from the base of the tongue inferiorly though the thyroglossal duct. This duct normally disappears following the development and migration of the thyroid gland, but if a portion of the duct remain it can form a thyroglossal cyst.