73

Etymology
From entry 73 in the Phillips Code, originally a list from 1 to 92 of numeric codes used to abbreviate common phrases used on telegraph lines.

Symbol

 * 1) The cardinal number seventy-three.

Symbol

 * 1)  regards, best regards (a valediction among ham radio enthusiasts).

Usage notes

 * 73s is occasionally used, but is etymologically incorrect, as 73 is already plural.