| focus, principal | <physics> The point at which a lens focuses an axial object point. Synonymous with focal point. (05 Aug 1998) |
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| Friend spleen focus forming virus | <virology> Defective virus found in certain strains of Friend virus, detected by its ability to form foci in spleens of mice and believed to be responsible in those strains for the production of a leukaemia associated with polycythaemia rather than anaemia. (18 Nov 1997) |