A dream was a series of sensations in the mind during sleep.
In late 2267, Captain James T. Kirk, while taking shore leave on Vulcan, dreamed that he and his brother Sam Kirk were visiting his grandmother's farm during an oppressively hot July, and that they would soon be off to the fishing hole. (TOS novel: The Vulcan Academy Murders)
In 2367, Captain Jean-Luc Picard dreamed that he was on a crowded shuttlecraft with his crewmates from the USS Stargazer in the aftermath of the Battle of Maxia, and that Jack Crusher was there as well, despite the fact that he had died some time earlier. (TNG novel: Reunion)