The period of time that the sample component resides in the stationary phase relative to the time it resides in the mobile phase. It is calculated from the adjusted retention time divided by the holdup time; k = (t_R – t_M)/t_M, where t_R is retention time for the sample peak and t_M is the retention time for an unretained peak. (Formerly, k´ was used, and it was called the capacity factor or the capacity ratio.)