passing light without diffusion or distortion | maximum clarity or distinctness of an image rendered by an optical system |
free from obscurity and easy to understand; the comprehensibility of clear expression | maximum clarity or distinctness of an idea |
| the concentration of attention or energy on something |
| a fixed reference point on the concave side of a conic section |
| a point of convergence of light (or other radiation) or a point from which it diverges |
| a central point or locus of an infection in an organism |
| special emphasis attached to something |