Final answer:
The combining form 'ocul(o)' is directly related to the eye, being used in terms referencing the anatomy and functions of the eyes essential for vision.
Step-by-step explanation:
The combining form ocul(o) is most closely associated with the definition 'eye'. It is derived from the Latin word for eye, 'oculus', and is used in various medical and anatomical terms relating to the eyes. Vision involves the eyes' ability to transduce light stimuli, which are protected by the eyelids and lacrimal system to keep the cornea clear from debris and abrasions.