1. An obsolete term for the dimness of eyesight, or a speck on the cornea of the eye: Since Jane couldn't see very well, she went to her ophthalmologist and was diagnosed to have achlys that was causing her poor vision.
2. A genus of butterflies in the subfamily Brassolinæ: One of the species of the genus Achlys is the gigantic owl-butterfly of South America, the C. eurylochus, which has a wingspan of about 9 inches!