Vein n. : One of the vessels which carry blood, either venous or arterial, to the heart. See Artery, 2.

Vein n. : One of the similar branches of the framework of a leaf.

Vein n. : One of the ribs or nervures of the wings of insects. See Venation.

Vein n. : A narrow mass of rock intersecting other rocks, and filling inclined or vertical fissures not corresponding with the stratification; a lode; a dike; -- often limited, in the language of miners, to a mineral vein or lode, that is, to a vein which contains useful minerals or ores.

Vein n. : A fissure, cleft, or cavity, as in the earth or other substance.

Vein n. : A streak or wave of different color, appearing in wood, and in marble and other stones; variegation.

Vein n. : A train of association, thoughts, emotions, or the like; a current; a course.

Vein n. : Peculiar temper or temperament; tendency or turn of mind; a particular disposition or cast of genius; humor; strain; quality; also, manner of speech or action; as, a rich vein of humor; a satirical vein.

Vein v. t. : To form or mark with veins; to fill or cover with veins.

Example:
What is a Vein that has popped out on my forehead?

Taxonomy
veins, blood, heart, body, deoxygenated blood, arteries, oxygenated blood, tissues, back to the heart, lungs, pulmonary veins, from the heart, hepatic portal vein, Venous insufficiency, Deep vein thrombosis, free encyclopedia, capillary beds, return blood.

Defination from the web:
vein is made of connective tissue, called tunica adventitia or tunica externa. ... Vein color is determined in large part by the color of venous blood, which is usually dark red as a result of its low oxygen content. ...