n. pl. pho·to·sen·si·tiv·i·ties
1. Sensitivity or responsiveness to light.
2. Medicine An abnormally heightened response, especially of the skin, to sunlight or ultraviolet radiation, caused by certain disorders or chemicals and characterized by a toxic or allergic reaction.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.