A pocket pen is literally a pen that is convenient for the pocket, but also describes the method of achieving a small carrying size whilst being larger in the hand when writing.
A pocket pen has a larger than normal cap, which conceals a large part of the pen when closed. On removing the cap it is placed on the back of the pen and thus creates a much longer barrel ready for writing. The two largest ranges of pocket pens are the Kaweco Sport (known in earlier years as the Kaweco Pocket pens) and the OHTO Tasche - 'tasche' being German for pocket (even though OHTO is Japanese!).
The Kaweco Sport range is determinedly retro in style, recreating a design whose origins go back to the 1930's. Their pocket range includes fountain pens in plastic and metal, plus rollerball pens and an unusual ink roller which uses fountain pen cartridges. There are also matching ballpoints and pencils, though these do not use the 'pocket' concept. Kaweco Sport pocket pens are 105mm long closed, 133mm when writing.
The more contemporary OHTO Tasche range on the other hand provides fountain pens, ballpoint pens, mechanical pencils and rollerballs, all with the same long-cap pocket design. Tasche items are around 98mm long closed and 145mm long when ready to write.
The pocket concept is one approach to making a pen compact to carry but larger to write with. The other main approach is the Telescopic Pen.