Qaisari/Jurjani
(Kasshaf Istilahat Al-Funun)
“Ta’ayyun is an entity/operation that allows for Imtiaz/Tashkhis (Discernment) of an object from all else that is not it.“

In modern English language Ta’ayyun means Ocular/Optical Contrast i.e. like a lens that goes in and out of  focus thus making high-contrast detailed or low-contrast blurred images. As we read later on, the contrast might not be optical in nature e.g. contrasting a thought from all thoughts, but it is optical-like meaning Ta’ayyun shares some attributes in its operations with an ordinary camera lens.