In medicine, phlebitis is an inflamed vein in a person's arm or (more commonly) leg. Mild phlebitis is nothing to worry about — it's treated with warm compresses and exercise.
Some people get phlebitis after being stuck in bed for a long time, or during a long plane flight that compromises the circulation of their blood. Phlebitis can be nothing more than a visibly swollen vein very close to the surface of the skin, but if it affects a deeper vein, it can be dangerous. This more serious phlebitis needs to be treated by a doctor. The Greek root of phlebitis is phleps, "vein."