Introduction
Mohini, "the enchantress", is a female form taken by Vishnu on at least two well-known occasions in the Puranas: to distribute the nectar of immortality justly to the gods after the churning of the ocean, and to trick the demon Bhasmasura into destroying himself.
The churning of the ocean
After the gods and demons together churned the ocean of milk and the physician Dhanvantari emerged carrying the pot of amrita, a quarrel broke out over who should drink it first. Vishnu appeared as the breathtakingly beautiful Mohini, and both gods and demons agreed to let her distribute the nectar. Through her skill, she gave it only to the gods, seated in one line, while distracting and delaying the demons, ensuring the gods alone gained immortality.
Bhasmasura
Mohini appears again to trick the Asura Bhasmasura, who had gained the power to burn anything he touched, into placing his own hand on his head during a dance, destroying himself; see the article on Bhasmasura for the full story.
Other appearances
In the Ramayana's Uttara Kanda, a union of Shiva and Vishnu-as-Mohini is, in some regional and later traditions, said to have produced the deity Ayyappan (Shasta), particularly in Kerala's devotional tradition, though this is not part of the earlier account in Valmiki.
Meaning
Mohini represents Vishnu's power to act through beauty, charm and cleverness rather than force, and is one of the clearest instances in Hindu tradition of a major male deity taking female form for a specific divine purpose.