#b# conventionally stands for the coefficient of the middle term of a quadratic expression.

The normal form of a generic quadratic equation in one variable #x# is:

#ax^2 + bx + c = 0#

Associated with such a quadratic equation is the *discriminant* #Delta# given by the formula:

#Delta = b^2-4ac#

The general solution of the quadratic equation may be written

#x = (-b +- sqrt(b^2-4ac))/(2a)#

or

#x = (-b +- sqrt(Delta))/(2a)#

Often people will assume that #a# is understood to be the coefficient of #x^2#, #b# the coefficient of #x# and #c# the constant term, and they will proceed directly from a quadratic equation such as #2x^2-3x+1 = 0# to speaking of something like #b^2-4ac# without telling you that #a=2#, #b=-3# and #c=1# are the coefficients.