Ian Andrew Troi II was an alien entity who wished to experience Human existence. In the form of a glowing white light, he entered into Deanna Troi and began his Human experience as a fetus. Troi stated that a glowing white light impregnated her while she was asleep. Dr. Katherine Pulaski confirmed that the fetus would reach full term within thirty-six hours. He was born in early 2365 and named after Deanna's father, Ian Andrew Troi.
At first, Troi did not realize that Ian was an alien. Jean-Luc Picard was initially worried that the child could present a danger, but he appeared to be completely harmless. He grew from an infant to an eight-year old in one day. At the same time, a plasma plague put everyone on the ship in danger. A contaminated specimen of the hazardous strain had begun to grow in the cargo bay, due to radiation from an unknown source.
Ian turned out to be the source of the radiation. When the young boy realized this, he decided to sacrifice his life to save the crew. As Troi looked on, Ian died, but not before he told her that he was the alien and that he wanted to experience the Human life cycle, including death. His body reverted to its original state and left the ship, but not before he had experienced Humanity, and Troi had experienced the joy and sadness of parenthood. (TNG: "The Child")
Ian was part of Commander William T. Riker's memories while being infected on the surface of Surata IV and treated in sickbay. (TNG: "Shades of Gray")
Appendices
Background
Ian Troi was played by R.J. Williams, at age 4 by Zachary Benjamin and by Josh Bell as an infant.
Influenced by the character of Ian Troi, a similar alien was briefly considered as a main character of Star Trek: Voyager, which was said to be born and then "matures rapidly." (A Vision of the Future - Star Trek: Voyager, p. 178)
Apocrypha
In the Star Trek: Destiny trilogy, Troi's attempts to conceive with Captain Riker after the two transfer to the Titan are hampered because carrying Ian caused previously-undetected damage to her reproductive system due to the unique radiation he generated. However, this damage is treated by the Caeliar, a highly-advanced race, resulting in Troi successfully carrying her daughter to full term.