FOCAL

focal

(adjective) having or localized centrally at a focus; “focal point”; “focal infection”

focal

(adjective) of or relating to a focus; “focal length”

Source: WordNet® 3.1

Etymology

Adjective

focal (not comparable)

Belonging to, concerning, or located at a focus

(medicine) limited to a small area

Noun

focal (plural focals)

(geometry, obsolete)

Source: Wiktionary

Fo"cal, a. Etym: [Cf.F. focal. See Focus.]

Definition: Belonging to,or concerning, a focus; as, a focal point. Focal distance, or length,of a lens or mirror (Opt.), the distance of the focus from the surface of the lens or mirror, or more exactly, in the case of a lens, from its optical center. --Focal distance of a telescope, the distance of the image of an object from the object glass.

Source: Webster’s Unabridged Dictionary 1913 Edition