If you're lightheaded, you're dizzy and faint, and you might need to sit down for a minute. The sight of enormous spiders makes some people feel lightheaded.
Standing up quickly can make you lightheaded, and you can also make yourself lightheaded by forgetting to eat lunch or blowing up a huge balloon with one breath. Being lightheaded, while usually not a serious condition, is typically caused by a temporary shortage of oxygen or blood getting to the brain. The word dates from the sixteenth century.