One of the five celestial trees in the garden of Indra - Mandaar, Paarijaat, Santaan, Kalp Vriksh, and Hari Chandan

It is commonly known as "Night-flowering Jasmine", or "Coral Jasmine" in English, or "Haarsingaar", or "Shephaalee" in Hindee language. Its leaves are opposite to each other. Its flowers are fragrant, white with yellow center and are produced in the cluster of 2-7 together. Every individual flower opens at dusk and finishes at dawn.

Note:
I read this in Wikipedia but I have never read this anywhere - "The tree was planted in the garden of Indra, the Lord of Heavens. As Krishn stole a branch of the tree, He was spotted by Indra. However, Indra desisted from placing a curse on Krishn since he was an incarnation of Vishnu. Still, Indra put forth a curse on the stolen branch that it will never bear fruit even though the flowers may bloom on the tree. Since the day the tree was planted at Baaraabankee (the wives' garden), it flowers but does not reproduce, because it has no seeds and the branch cannot take its root."

According to Indian scriptures it is a Divine tree - Coral tree with crimson flowers. It came out from Saagar Manthan. As it came out of the sea, Indra took it and planted it in Swarg Lok. Its flowers never wilt. Bhaagvat Puraan 10/u1 says that when Krishn got Dwaarakaa city, Indra sent Sudharmaa Sabhaa and this Paarijaat tree there and they remained there till Krishn lived on Prithvi. As Dwaarakaa sank into the sea, both Paarijaat tree and Sudharmaa Sabhaa returned to Indra Puree.

Nyctanthes arbor-tristis (sometimes incorrectly cited as Nyctanthes arbortristis or Nyctanthes arbor tristis) is commonly known as
* Night-flowering Jasmine
* Coral Jasmine
* Paarijaat (also spelt as Paarijat or Paarijaata)
* Harsingar or Haarsingaar
* Shephali or Shefaalee
* Maramalli or Pavazha malli in Tamil (also spelt as pavaza malli or pavala malli)

The tree is sometimes called the "tree of sorrow", because the flowers lose their brightness during daytime; the scientific name arbor-tristis also means "sad tree". The flowers can be used as a source of yellow dye for clothing. The flower is the official flower of the state of West Bengal, India, and for Kanchanaburi Province, Thailand.

Paarijaat in Mythology
Paarijaat appears in several Hindu myths. In one myth, which appears in Bhaagvat Puraan, the Mahaabhaarat and the Vishnu Puraan, Paarijaat appeared as the result of the Saagar Manthan (Churning of the Milky Ocean).

In another myth -- Once a royal princess fell in love with the Sun god - Soorya Dev. She was enamored of his brilliance and beauty as he daily passed through the sky from East to West in his fiery chariot. Her devotion attracted his attention and for a while he favored her with his attention but after a while he was distracted with other interests and she was deserted. In despair she killed herself and from her cremated ashes this Paarijaat tree arose. Since she was rejected by Soorya Dev, the flowers of the tree only bloom at night. Then before the Sun rises the flowers fall so his rays will not strike her. Based on this story the tree was given the species name earbortristisi which means the tree of sorrow.

After the Paarijaat tree emerged from the ocean of existence it was taken to the Heaven and planted in the pleasure garden of Indra - the Lord of the gods. One day Naarad Muni visited this garden in his meditations and saw this lovely tree emitting its divine perfume. Using his Yaugik powers he gathered up some of these ethereal blossoms and brought them back to the physical plane and gave them to Rukminee, the favorite wife of the renowned Avataar of Vishnu, Lord Krishn, who was at that time dwelling in Dwaarakaa in western India . The flowers were so lovely and the fragrance so delicate that Satyabhaamaa, another wife of Lord Krishn became desirous of possessing that celestial tree and had it planted in her own garden. She was jealous of the attention Krishn was showing to Rukminee and wanted to give more time to her. She implored Him to obtain the tree for her. To satisfy her desire He entered into a state of deep meditation and in that state plucked up the tree from the garden. Before leaving that place He was accosted by the keepers of the garden and was told he would incur the wrath of Indra as the tree belonged to his wife Shachee. But Satyabhaamaa would not be put off by any obstacle and said that the tree was the common property of all and had as much place on Earth as it had in the Heavens. As a result Krishn waged a great war with Indra and his celestial army.

In the end Krishna's strength prevailed and Indra was forced to retreat. At that time Satyabhaamaa taunted him as being a coward but decided to give back his celestial tree. Krishn also consented to return the tree to its celestial abode. But Indra said that there was no shame in being defeated by the Avataar of Vishnu and that the tree should be taken to earth and planted in Dwaarakaa where its fragrance could be enjoined by all the people of the earth. Thus the first Paarijaat tree was planted and its divine fragrance was said to spread for three furlongs. Its aroma was charged with so much power that it would help people inhaling it to remember events of their past lives.

In this instance we can also see that the sages were explaining to the people that fragrance was a valuable means of accessing stored memories. In the East memory has a much more comprehensive meaning than in the West as it can include past lives as it is believed that the soul takes countless births on its journey to perfection but the same basic principal is explained in this story as is encountered in western literature regarding how fragrance stimulates memories of past events of one's life.

Paarijaat as Medicine
The fresh leaves of the plant prepared in the form of juice, infusions or decoctions and in combination with other herbs are found to be useful in treating inflammations, sciatica, pruritis (itching), fever, bronchitis, asthma, cough, dyspepsia (difficulty with digestion associated with pain, flatulence, heartburn and nausea), constipation as the active principals contained within them are found to have anti-bacterial, anti-inflammatory, digestive, expectorant, sudorific, diuretic and laxative properties. The leaves were also considered antidotal for reptile venom. Various preparations of the fresh flowers were found useful in treating diseases like colic, dyspepsia, flatulence, graying of hair and baldness as they were astringent, stomachic, and carminative in nature. The bark when chewed with betel nut and leaf was eaten to promote expectoration of thick phlegm. The seeds when powdered and prepared as a paste are used to cure scurvy and affections of the scalp.