Unilateral tolerance is single side tolerance.bilateral tolerance is double side tilerance

Unilateral tolerance is a variation on one side either +ve or -ve while as bilateral is a variation on both side +very and -ve

When the two limit dimensions are only on one side of the normal size (either above or below) the tolerance are said to be unilateral whereas, when the two limit dimensions are above and bellow normal size ,(i.e. on both side of the normal size) the toleranceare said to be bilateral