Webster's English Dictionary

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focus

1. fo.cus \'fo--k*s\ \-.si-\ n or fo.cus.es or fo.ci [NL, fr. L, hearth] pl 
    1: a point at which rays converge or from which they diverge or appear to 
   d iverge; specif : the point where the geometrical lines or their 
   prolongations conforming to the rays diverging from or converging toward 
   another point intersect and give rise to an image after reflection by a 
   mirror or refraction by a lens or optical system 2a: FOCAL LENGTH  2b: 
   adjustment for distinct vision; also : the area that may be see n 
   distinctly or resolved into a clear image 3: one of the points that with 
   the corresponding directrix defines a conic  section 4: a localized area of 
   disease or the chief site of a generalized disease o r infection 5: a 
   center of activity, attraction, or attention  6: the place of origin of an 
   earthquake  : having or giving the proper sharpness of outline due to good 
   focusing  : not in focus  - in focus 
2. focus vb or fo.cused also fo.cussed;  also fo.cus.ing;  also fo.cus.sing 
   1: to bring to a focus : CONCENTRATE  2: to cause to be concentrated  3a: 
   to adjust the focus of  3b: to bring into focus  1: to come to a focus : 
   CONVERGE  2: to adjust one's eye or a camera to a particular range