What is defined as an opening or hole that allows a portion of light from a source to enter?

The term that refers to an opening or hole that allows a portion of light from a source to enter is "aperture." In optics and photography, the aperture is significant because it controls the amount of light that reaches the sensor or film. A larger aperture lets in more light, which can be beneficial in low-light conditions or for achieving a shallow depth of field in images. This definition aligns with the characteristics of an aperture, which is fundamental to understanding how various optical devices function.

The other terms mentioned have different meanings and functions. A lens is a piece of transparent material, usually glass or plastic, that focuses or disperses light. A prism is an optical element that refracts light, separating it into its constituent colors. Focus refers to the point where light rays converge or diverge to form a clear image. Each of these plays a vital role in optics but does not specifically define an opening for light entry like an aperture does.