Origin of the word:

1640s, "point of convergence," from Latin focus "hearth, fireplace" (also, figuratively, "home, family"), which is of unknown origin. Used in post-classical times for "fire" itself; taken by Kepler (1604) in a mathematical sense for "point of convergence," perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before Kepler, but it is not recorded). Introduced into English 1650s by Hobbes. Sense transfer to "center of activity or energy" is first recorded 1796.

Definition:

noun

1. the center of interest or activity.
2. the state or quality of having or producing clear visual definition.

verb

1. adapt to the prevailing level of light and become able to see clearly.
2. pay particular attention to.
3. place the focus on (an element of a sentence).
    
4.  

Other useful definitions by Creative, and Game-Changing Thinkers:

"As with Choice, people tend to think of focus as a thing. Yes, focus is something we have. But focus is also something we do." - Essentialism by Greg McKeown

"Focus
Minimize the circumstances that can break your focus.
Your focus causes discipline.
Discipline leads to credibility. 
Credibility to trust." gabe@gabethebassplayer.com