错觉（illusion）是感觉的扭曲。是大脑对刺激的错误分析。在心理学研究中，意指假性幻觉所呈现的状态。当一个人将畏惧或焦虑的感受投射在外在物体与经验上，或是因为想像、虚构或错误的联想而产生畏惧或焦虑的感觉，就会产生假性幻觉。大部分人都会有错觉的。最常见的错觉是视觉上的错觉。

以下是视觉上的错觉例子。

值得一提的是，错觉和幻觉是两种完全不一样的现像。前者是大脑对刺激的分析错误，后者是在完全没有刺激的情况下，大脑仍然感受到刺激。

举一个例子，一个人听到有人在叫他，但事实上并没有任何人在叫他。（这是幻觉） 一个人听到水流动的声音，但事实上，该声音是其他相似于水流动的声音，并非真正的水流动的声音。（这是错觉）

An illusion is a distortion of the senses, revealing how the brain normally organizes and interprets sensory stimulation. Though illusions distort reality, they are generally shared by most people. Illusions may occur with any of the human senses, but visual illusions (optical illusions), are the most well-known and understood. The emphasis on visual illusions occurs because vision often dominates the other senses. For example, individuals watching a ventriloquist will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words. Some illusions are based on general assumptions the brain makes during ****. These assumptions are made using organizational principles (e.g., Gestalt theory), an individual's capacity for depth pe****tion and motion pe****tion, and pe****tual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside of the body within one’s physical environment.