True dip is the angle between an inclined surface and the horizontal, measured perpendicular to a horizontal line in the plane (strike). The dip seen in a limited data set (well bore, outcrop, cross section, seismic line, etc.) may not be perpendicular but rather cut at an angle, and is the apparent dip; this is less than the true dip. It is usually written as 4S (dipping 4 degrees in the southerly option perpendicular to strike; this requires that a strike be given), 4 SE (likewise), or 4 65SE (dipping 4 degrees in a 65SE direction; this by inference gives the strike, because it is perpendicular).
Figure 1 illustrates the strike and dip of beds.