A pen cancellation
In philately, a pen cancel is a cancellation done with pen or marker. In the early, pre-ballpoint pen days of adhesive stamps it took longer to apply a pen cancel than a handstamp, which led to the development of handstamped cancellation devices. According to Linns.com, "A pen cancel may also indicate that a stamp was used as a fiscal".
Sometimes philatelists may have trouble distinguishing between a pen cancel and a pen marking on a stamp for another reason.
Pen cancels are sometimes seen today, as, in order not to lose revenue through stamp reuse, postal employees are instructed to obliterate (through use of pen-cancellation, among other possible methods), stamps missed by the cancelling machine. Depending on the type of pen or marker used, a pen cancel may destroy the value for stamp-collecting purposes of a used stamp, or the cover if it is saved on cover, and this has led to complaints by stamp collectors that postal employees accused of overzealousness have applied overly thick and enthusiastic marker cancels.