In a simple sense, an illusion is when something isn't what it seems. Our brain likes to play tricks on us and tell us that things we are seeing that are not physically real are real. What is happening in this situation is our brain are perceiving an alternate reality to what is physically real. These illusions occur because our sensory system is not perfect. There are gaps in its accuracy that lead to our brain filling in these holes and making some things appear real. Most of the time our sensory system is right and does a great job of filling in the gaps accurately, but sometimes it doesn't. Our brain's interpretation of what is physically present can be faulty.
The photo included in this posting is a great example of an illusion. The photo is either a girl or a man playing a saxophone. Our brain tries to piece together what we are seeing and either determines that it is a women or the saxophone player. If you look hard enough you will eventually see both images. There are images like these that we have all interacted with at least once in our life, but there are also illusions we can barely detect. A simple one would be a mirage. As a kid I always saw these on the streets and I was fascinated by them. As a child I always thought they could be real until we would drive by them and I would find out that it really was an illusion.
It is important to be aware of illusions because if we are not careful and let our brains deceive us we could get hurt or in trouble. In the rare chance that you were stuck in a desert and did not know about illusions you might actually try and run after a mirage.
After reading about this concept I wondered about how a researcher can detect what is an illusion. Some illusions are obvious, but I feel that if all humans are prone to experiencing illusions there might be some that are hard to detect. I understand that there are strict scientific studies to identify illusions, but I still feel skeptical that we have discovered all the illusions in our world.