What phenomenon occurs when the eyes fail to maintain parallelism?

The phenomenon that occurs when the eyes fail to maintain parallelism is known as strabismus. This condition is characterized by a misalignment of the eyes, where one eye may turn inwards, outwards, upwards, or downwards relative to the other eye. Strabismus can result from various factors, including muscle imbalances, neurological issues, or refractive errors. It is important to recognize strabismus because it can lead to complications such as double vision (diplopia) or amblyopia (lazy eye) if not treated.

In contrast, the other terms refer to different visual conditions. Diplopia refers specifically to double vision, which can result from strabismus but is not the phenomenon of misalignment itself. Presbyopia is the age-related loss of the eye's ability to focus on nearby objects, and myopia, or nearsightedness, is a refractive error where distant objects appear blurry while close objects are seen clearly. These conditions do not pertain to the parallelism of the eyes but rather to how the eyes focus or process visual information.