Grahan means eclipse. Literally speaking, the eclipse is a Latin/Greek word Eclipsis / Ekleipsis / Ekleipein, meaning abandonment, fail to appear, obscuring (covering) one celestial body (star) by another, a temporary or permanent dimming or cutting off light, a disgraceful or humiliating end or downfall, to overshadow or surpass, to hide from view, to block passage of all or part of one celestial body into the shadow of another. In Sanskrit it is called Grahan, meaning held forcibly or to seize or afflicted, because of being captured. It also means to absorb or swallow. It also means to accept.

Scientifically it is the disappearance, partially or wholly, of the Sun when the Moon comes in between the Earth and the Sun (called Solar eclipse) or of the Moon when the Earth's shadow falls across it. It is said that during eclipse, there is down pouring of energy onto the Earth from the Universe, in the form of eclipse. Eclipses are cause by the special alignment of the Sun, the moon and the Earth in different ways. In Lunar eclipse, the Earth blocks the light of the Sun falling on the Moon which makes it disappear on the Earth; in Solar eclipse, the Moon blocks the light of the Sun falling on the Earth from some parts of the Sun.

The Lunar eclipse falls on the Full Moon Day when the Earth blocks the light of the Sun from the Moon and the Solar eclipse occurs on the New Moon Day when the Moon blocks the the light of the Sun coming to the Earth. On New Moon Day the Sun and the Moon are in conjunction as seen from the Earth. In this situation the Moon is the eclipsing body and passing between the Earth and the Sun while casting a traveling shadow across Earth's lighted surface, so that the Sun is partially or wholly obscured.

Types of Solar and Lunar Eclipses
There are four kinds of Soorya Grahan (Solar eclipse).
(1) Total eclipse - when the Sun is totally obscured by the Moon, they are rare events, though they occur.
(2) Annular eclipse - when the Sun and the Moon are exactly in line, but the size of the Moon is apparently smaller than that of the Sun.
(3) Hybrid eclipse - It is the intermediate between the Total and the Annular eclipses.
(4) Partial eclipse - when the Sun and the Moon are not exactly in line and the Moon only partially obscure the Sun.

There are Three Types of Chandra Grahan (Lunar eclipse), listed as below:
(1) Poorn Graas - Total Lunar Eclipse (Completely passing through Umbra Zone)
(2) Khand Graas - Partial Lunar Eclipse (Partially passing through Umbra Zone)
(3) Madhya Graas - Also known as Chhaya Kalpa Graas (Passing through Penumbra Zone)

As per Dharm Shaastra (Kaal Nirnaya also supports this), during Chhaayaa Kalp Grahan, the eclipse rules need not to be followed. Nevertheless, there is not harm in doing Saadhanaa during this period too........'More' gives 'More' fruits.

Reference of Eclipses in Hindu Religious Books
Their references may be found in  Mahaabhaarat, Bhaagvat Puraan, and other Vishnu related Puraan. According to Hindu mythology, the Solar and the Lunar eclipses occur because Raahu and Ketu swallow the Sun and the Moon respectively on those days. (Read their story here - this story comes in Bhaagvat Puraan, 8/3.

Another reference of the "Sarvgraas" Solar eclipse is made in Bhaagvat Puraan, 10/u19 and Krishn goes to take a holy dip along with His relatives and friends.

In Mahaabhaarat, the reference to Solar eclipse is made in -----  when two eclipses fall within the gap of 13 days during the course of Kurukshetra war.

In Vaalmeeki Raamaayan, a description of Total Solar eclipse is given, and a mention of Raahu has been made in the context of war between Raam and Khar.

In the Nav Graha Stotra, supposed to be written by Ved Vyaas Jee, the description of Raahu is well suited to the reference of a Solar eclipse.

Astrological Importance
Astrologically Raahu and Ketu are the two points on the intersection of the paths of the Sun and the Moon as they move in the celestial sphere - North and South Lunar Nodes or Ascending and Descending Nodes. In fact these eclipses occur when the Sun and the Moon are on one of these points. In Ashtottaree Dashaa system there are only eight planets including Raahu while in Vinshottaree Dashaa system there are nine planets - the ninth being Ketu.

Astrologically the Solar eclipse is not considered good and brings many calamities in the area where it occurs. In Mahaabhaarat Vyaas Jee tells Dhritraashtra about the two eclipses falling within 13 days and advises him to stop the war.

Spiritual Importance of Eclipse Period
The entire period of eclipse, Solar or Lunar, is said to be the Parv Kaal or scared period. Most people do prayers, Poojaa, Purashcharan (recitation of Mantra), Pitri Tarpan, Daan etc during this time. Bath at the beginning of the eclipse and above written activities during the eclipse time and then again bath after the eclipse is complete are very sacred and give multiple effects. Chanting the Vaidik Mantra or Stotra ward off all kinds of negative energies which attack during the eclipse time.

Some of the useful prayers to be recited at this time are - Gaayatree Mantra, Vishnu Sahastra Naam, Vishnu Panjar Stotra, Raaghavendra Stotra, Shree Nrasinh Kavacham, Durgaa Kavach, Naaraayan Ashtaaksharee, Shiv Panchaaksharee, Aaditya Hridayam (Solar eclipse), Shree Krishn Ashtakam (Lunar eclipse).

When the Solar eclipse falls on Sunday Amaavasyaa, and Lunar eclipse falls on Monday Poornimaa, they have a special importance on those days. When a Solar eclipse falls on a Monday, it is also auspicious as it falls on Somvatee Amaavasyaa. This day is also called Maunee Amaavasyaa as Saadhu keep Maun (silence) on this day.

Traditions and Beliefs
This period is otherwise regarded as inauspicious (Vedhaa) for doing anything else other than the above mentioned activities. This inauspicious period begins 4 Prahar (12 hours) before Solar eclipse and 3 Prahar (9 hours) before Lunar eclipse. Prahar is 1/8th of a day and night combined (24 hours).

In Hindu traditions, people do not cook or eat food during this period. Old people, sick people, pregnant women, children etc are exempted from this rule.

If the Solar eclipse ends after the sunset, people do not eat during the night, but eat in the morning after the Sun has risen.

People take bath before the Solar eclipse has begun and after the Solar eclipse has ended. If any sacred river is available, it is better to take a dip in that waters.

During the eclipse period they perform above mentioned activities.

Kush (Kushaa) grass is kept near the milk, yogurt, pickles, and stored water during the eclipse as a remedy.

If a death anniversary falls on the day of eclipse, its Tarpan is performed after the eclipse is over.

Pregnant women sit inside the room where no sunlight enters the room to avoid congenital deformities in the fetus.

People do not watch the Solar eclipse with naked eye. One should use dark sunglasses to see it.

People avoid sleeping and nature calls during this period.

Some Strange Facts About Eclipse
It is believed that similar type of eclipse occur once in 18/19 years. It is also believed that events repeat during that time.

It is believed that the effect of the eclipse will remain till the time of eclipse - hour, minutes and seconds corresponding to the years, months, and days.

Damage to eyes may be caused by invisible infra-red energy from the Sun's rays during Solar Eclipse.

People do not take or cook food during this time because they believe that some most harmful invisible Ras (juices or matter) can be absorbed by the food and the food can get contaminated. Vikram Sarabhai Research Center experimented to keep Kush grass near the water in July 2008 Solar eclipse time and it did not get contaminated.

The longest possible Total SolarEclipse time is only 7 minutes and 40 seconds near the Equator. The June 1955 Solar eclipse lasted for 7 minutes 4 seconds, which was said to be the longest duration during the last 1,238 years, and a similar eclipse occurred on July 11, 1991.