Noun

1. The concentration of attention or energy on something
2. Maximum clarity or distinctness of an image rendered by an optical system
3. Maximum clarity or distinctness of an idea
4. A central point or locus of an infection in an organism
5. Special emphasis attached to something
6. A point of convergence of light (or other radiation) or a point from which it diverges
7. A fixed reference point on the concave side of a conic section

Verb

1. Direct one's attention on something
2. Cause to converge on or toward a central point
3. Bring into focus or alignment; to converge or cause to converge; of ideas or emotions
4. Become focussed or come into focus
5. Put (an image) into focus