California Code, Insurance Code - INS § 6045

(a) “A city or town block” is an area of not more than one hundred sixty thousand square feet having at least one frontage in a closely built up district fronting on a used public highway, surrounded on all sides by a clear space at least equal in width to the clear space of such public highway.

(b) “Closely built up district” means territory abutting a public highway where for not less than a quarter of a mile the buildings average less than one hundred feet apart.

(c) “One risk” means one hazard under one or more policies, subject to one fire and relates to the amount named in the policy or policies.

(d) “Clear space” means space free from combustible material likely to communicate fire.