Invisible ink
（重定向自Sympathetic ink）
![A coded letter from Benedict Arnold, originally written in invisible ink. Handwriting by Peggy Shippen Arnold is interspersed with coded communication in Arnold's hand.](/Images/godic/202502/14/ArnoldCipherLetter3004.jpeg")
![Detail of a software user manual security feature made with invisible ink.](/Images/godic/202502/14/SAM_04203005.jpg")
Invisible ink, also known as security ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means. Invisible ink is one form of steganography.