What is Retention?

The retention time $t_R$ of a compound is the time from injection to the maximum of its peak; the hold-up time $t_M$ is the retention time of an unretained compound, which never enters the stationary phase. The retention factor $k$ of a compound on a column is

It is not the $R_f$ of a thin-layer plate, which measures a distance travelled, though both describe the same partition.