幻覚;げんかく: a hallucination - whether a pink elephant or an abstract geometric form.

幻:まぼろし: like 幻覚 but more broad - it can be sounds, tastes, etc.

幻想:げんそう: a vision - a hallucination that goes on and on like a movie. Like the English word 'vision,' 幻想 is often used to tell the future or help making a decision.