Bond Basis

A type of day count convention specific to bonds, the bond basis is a factor used to calculate accrued interest on a bond since the last coupon payment. For the bond basis, the fraction of days in a month over days in a year is 30/360, a fraction which, while slightly inaccurate, is easy to use for calculations. The day count convention for treasury bonds however, is actual / actual. 

Wikipedia: http://en.wikipedia.org/wiki/Day_count_convention