Phototonus is a term that you might not be familiar with, but it refers to the sensitivity of an organism or a cell to light. There are many synonyms for phototonus that you can use to describe this characteristic. These include photosensitivity, photosensitivity reaction, photoreactivity, photosensitization, photosensory response, and phototropism. Each of these terms highlights a slightly different aspect of phototonus, such as the ability to detect light, the reaction to light, or the way an organism or cell grows in response to light. No matter which term you choose to use, they all describe the important role that light plays in the function and behavior of living things.