a pleasant daydream
reverie - daydream
If you are lost in thought, or find yourself in a pleasant state of detachment imagining nice things, you are engaged in a ‘reverie’. Though it may sound like something positive, it can be problematic in situations where attention is required and necessary, such as in the workplace or when driving a car.
Alice was lost in her own world during the meeting, so I nudged her to snap her out of her reverie.
On my walk, I was lost in reverie until I walked straight into a lamppost!
I was startled out of my reverie by the sound of my phone ringing.