vigil

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A watch kept during normal sleeping hours.
• noun The act or a period of observing; surveillance.
• noun The eve of a religious festival observed by staying awake as a devotional exercise.
• noun Ritual devotions observed on the eve of a holy day.

from The Century Dictionary.

• noun The act of keeping awake; abstinence or forbearance from sleep at the natural or ordinary hours of rest; the state of being awake during the natural time for sleep; sleeplessness; wakefulness; watch: commonly in the plural.
• noun Devotional watching; hence, devotions, services, praise, prayer, or the like performed during the customary hours of sleep; nocturnal devotions: commonly in the plural.
• noun Eccles.: Originally, in the early church, the watch kept in a church or cemetery on the night before a feast, the time being occupied in prayer.
• noun Hence— The day and night preceding a festival; the eve or day before a festival; strictly, an eve which is a fast.
• noun A wake.

from the GNU version of the Collaborative International Dictionary of English.

• noun Abstinence from sleep, whether at a time when sleep is customary or not; the act of keeping awake, or the state of being awake; sleeplessness; wakefulness; watch.
• noun Hence, devotional watching; waking for prayer, or other religious exercises.
• noun Originally, the watch kept on the night before a feast.
• noun Later, the day and the night preceding a feast.
• noun A religious service performed in the evening preceding a feast.
• noun (Bot.), [R.] a peculiar faculty belonging to the flowers of certain plants of opening and closing their petals at certain hours of the day.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A watch kept during normal sleeping hours, especially over the body of a recently deceased or dying person.
• noun A period of observation or surveillance.
• noun The eve of some religious festival in which staying awake is part of the ritual devotions.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a purposeful surveillance to guard or observe
• noun a period of sleeplessness
• noun the rite of staying awake for devotional purposes (especially on the eve of a religious festival)