Reality was the quality or state of things and events having existence and being real.
Sentient species, such as Humans, have the ability to distinguish between thought and reality. (TNG: "Where No One Has Gone Before")
In the Talosian community, as they concentrated on developing their mental powers, their dreams became more important than reality. (TOS: "The Cage")
In 2254, when he was experiencing an illusion of being back home in Mojave with Vina, Captain Christopher Pike recognized it as an escape from reality. It was a life free of responsibilities and frustrations. He came to understand what Doctor Phil Boyce had said about living. A person who confronted life lived, and a person who escaped from life died. (TOS: "The Cage")
In 2364, despite the holographic Minuet being a computer-generated image, her smell, touch, the way she felt, and even the things she said and thought seemed so real to William T. Riker. After Riker asked how real she was, Minuet responded that she was as real as Riker needed her to be. (TNG: "11001001")