Emptiness was a feeling that one was going through life mechanically, devoid of emotions and purpose, and therefore was empty inside.
In 2267, Lazarus described "a thing. All white, black and empty. A terrible emptiness." (TOS: "The Alternative Factor")
Adam's song "Hey, Out There!", performed in 2269, referenced "a mile wide emptiness between you and me". (TOS: "The Way to Eden")