thread vein 
noun
A very slender vein, especially one on the face that is visible through the skin.I can remember having a small thread vein that looked a bit like a blue map under the skin....
- Telangiectasia are also referred to as spider veins, star bursts, thread veins, or matted veins.
- Small patches of brown skin discolouration or areas of thread veins form where the veins were removed.