What does the ABC stand for in standard form Ax+By=C?

What does the ABC stand for in standard form Ax+By=C?

Ax+By+C is a generalized form (in fact the standard generalized form) for a linear equation
where A, B, and C are placeholders for constants (the x and y are variables).
The generalized form, Ax+By+C, includes equations such as
$color(white)(XXXXX)$$7x+3y=98$
$color(white)(XXXXX)$$(1)x+(-5)y=14$
$color(white)(XXXXX)$$9261x+83y=-7491$
$color(white)(XXXXX)$$color(white)(XXXXX)$any infinitely many others
By normal usage A should be an integer value which is not negative.