olemus
(1a) optikas:
tulipunkt, kiirte koondumise punkt
Wiktionary:
1. (countable, optics) a point at which reflected or refracted rays of light converge
https://upload.wikimedia.org/wikipedia/commons/e/e9/Geometric_Optics_focus.PNG
https://en.wikipedia.org/wiki/Focus_(optics)

(1b) geomeetrias:
mõnede kõveraliikide (näiteks koonuselõigete)
määramise tugipunkt
= a reference point for defining some types of curves
https://www.mathsisfun.com/definitions/focus.html
https://en.wikipedia.org/wiki/Focus_(geometry)