Definition, according to the Oxford English Dictionary: the fact of experiencing some things in a different way from most other people, for example experiencing colors as sounds, experiencing shapes as tastes, or feeling something in one part of the body when a different part is stimulated.
Allen-Hermanson, Sean, and Jennifer Matey. “Synesthesia.” Internet Encyclopedia of Philosophy, Internet Encyclopedia of Philosophy, https://iep.utm.edu/synesthe/#:~:text=The%20word%20%E2%80%9Csynesthesia%E2%80%9D%20or%20%E2%80%9C,a%20union%20of%20the%20senses.
Informal Definition, the sensation of experiencing a sense caused by a stimulus that is not associated with that sense, an example that best illustrates this feeling is the Bouba/Kiki effect where shapes were presented (shown below), and asked which ones they associated with the sound “Bouba” and the sound “Kiki”. Americans, Tamil speakers, children and infants all consistently named the spiky shape “Kiki”, while they named the round shape “Bouba”.
Etymology, from the Greek root syn, meaning union, and aesthesis, meaning sensation, thus the word synesthesia is directly translated from Greek as a union of the senses.
Hornby, Albert Sydney. Oxford Advanced Learner's Dictionary of Current EnglishA/ [by] A.S. Hornby ; Editor Jonathan Crowther. Oxford, England :Oxford University Press, 1995.