Focus: concentration, attention.
Focus is the Latin word for “fireplace.” In classical times, the fireplace was considered to be the center of the household. It not only served as the kitchen stove, where all the meals were cooked, but also as the central heating system. It was often the only source of light at night, as well. The fireplace was the heart of the home, the place were everyone would gather, the focus of all activity.
Astronomer Johann Kepler, one of the first scientists, brought the term focus into the modern world. In his book on optics (Astronomiae Pars Optica), he named the point where the rays of light coming through a lens converge the “focus.” This was because sunlight passing through a lens converges on a single point hot enough to cause a fire. So the focus is a literal fireplace where the sun’s rays cause ignition. Focus in this sense was brought into English by Hobbes (1650).
Our use of the term focus as a synonym for concentration is an analogy: the mind’s scattered “rays” are converged onto a single point.