The medical term vasovagal syncope refers to a drop in blood pressure leading to fainting or just dizziness and caused by a variety of factors, all encompassed by the term vasovagal which means a vas(cular) effect involving the vag(us) nerve (which means they don't really know what causes it). I have such a condition and when walking in Paris next month, I would like to be able to tell passerbys what's wrong with me if need be. The treatment, by the way, is just to lie down until you get better (hooray for modern medicine).