I find following defination of Diwali at wikipedia.org
Diwali, or Deepawali is a major Indian festival, significant in Hinduism and Jainism and Sikhism. Celebrated by Hindus,Jains and Sikhs across the globe, as the “Festival of Lights,” where the lights or lamps signify the uplighting of darkness and victory of good over the evil within. The day also commemorated the homecoming of King Rama of Ayodhya, after a 14-year exile in the forest, and thus the people of Ayodhya (the capital of his kingdom) welcomed him back by lighting up rows of lamps (deepa-wali), thus its name, Deepawali, or simply shortened as Diwali.
The celebrations focus on lights and lamps, particularly traditional dīpa or deeya (earthen lamp), and fireworks. Though colloquially called Diwali in North India, in South India it is called Deepavali.
Lakshmi Puja is performed in the evening to seek divine blessings of Goddess of Wealth. Diwali gifts are exchanged among all near and dear ones.