Transillumination: The passing of a strong beam of light through a part of the body for medical inspection. A method of examination by the passage of light through tissues or a body cavity.
A common use of transillumination is in infancy. The infant's skull should normally not transilluminate (let a beam of light pass through it). But in hydranencephaly, a condition in which the cerebral hemispheres are almost totally absent, the head appears normal, but when transilluminated, light shines completely through.
This is just one example of the use of transillumination, a simple, inexpensive, painless procedure.