Mental pictures

nightmare, apparition, hallucination, vision, daydream, fantasy, wraith, specter, incubus, image, illusion, mirage, trance, idea, impression, reverie, romance, dream life, evidence of the unconscious, flight of fancy, castle in the air, castle in Spain, air castle; see also fantasy 2, illusion 1, thought 2, vision 3, 4.

Antonyms reality*, verity*, truth. *

A fond hope

aspiration, goal, desire, ideal; see hope 2, purpose 1.

Unattainable idea

chimera, bubble, will-o'-the-wisp, pipe dream*; see sense 1; fantasy 2, illusion 1.

To have visions, usually during sleep or fever

be delirious, have nightmares, see in a vision, have flashes, hallucinate, fancy, visualize, envisage; see also visualize.

To entertain or delude oneself with imagined things

fancy, fantasize, daydream, indulge in reveries, imagine, conceive, have notions, conjure up, create, picture, visualize, muse, stargaze, moon, idealize, be up in the clouds, be on cloud nine, build castles in the air, build castles in Spain, go woolgathering, be moonstruck, let one's mind wander, talk through one's hat*, pipe-dream*, blow bubbles*, search for the rainbow's end, look for the pot of gold; see also imagine 1.

To conceive mentally; usually used in the negative

think of, consider, believe, suppose; see assume 1, consider 3.