**Core Concept**
Incomitant squint, also known as incomitant strabismus, refers to a type of strabismus where the angle of deviation between the eyes varies in different directions of gaze. This occurs due to a problem with the extraocular muscles or their innervation.

**Why the Correct Answer is Right**
Incomitant squint is characterized by a change in the angle of deviation with different directions of gaze, which indicates a problem with the extraocular muscles or their innervation. The medial rectus muscle is responsible for adduction, the lateral rectus muscle for abduction, the superior rectus muscle for elevation, and the inferior rectus muscle for depression. In incomitant squint, the deviation is not constant in all directions of gaze, suggesting a weakness or paralysis of one or more of these muscles.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incomitant squint can be caused by a variety of factors, including paralysis of the extraocular muscles, but it is not exclusive to cranial nerve III palsy. While cranial nerve III palsy can cause incomitant squint, it is not the only cause.
* **Option B:** Esotropia is a type of strabismus where the eyes turn inward, but it can be either comitant or incomitant. Comitant esotropia has a constant angle of deviation in all directions of gaze, whereas incomitant esotropia does not.
* **Option C:** Incomitant squint is indeed more common in children, but this statement does not directly address the definition of incomitant squint.
* **Option D:** The definition of incomitant squint is not related to the presence of nystagmus.

**Clinical Pearl / High-Yield Fact**
Incomitant squint can be differentiated from comitant squint by the change in angle of deviation with different directions of gaze. It is essential to assess the deviation in multiple directions to accurately diagnose incomitant squint.

**Correct Answer:** B.