entrance pupil

noun

: the stop in an optical instrument or the virtual image of the stop that determines the angular diameter of the bundle of rays traversing the instrument from a given point in the object space compare exit pupil

The Ultimate Dictionary Awaits

Expand your vocabulary and dive deeper into language with Merriam-Webster Unabridged.

1. Expanded definitions
2. Detailed etymologies
3. Advanced search tools
4. All ad-free

Discover what makes Merriam-Webster Unabridged the essential choice for true word lovers.

Start Your Free Trial Now