It is usually called imprint the inscription that generally shows the name of the manufacturer together with other data, which are traditionally found "imprinted" on the pen, in the most common case in the longitudinal direction on the body of the same as in the adjacent figure. The custom of having an imprint was practically universal until the '20s and on hard rubber models, to disappear gradually on models in plastic or metal. A gallery of several imprint photos can be found here.
In addition to a reference to the manufacturer, an imprint can contain various other information, such as the name of the model, the country of production, any logos used by the company, references to patents used for the pen, etc. In addition to the illustrated case of longitudinal arrangement, there are imprint placed circularly on the bottom of the pen (in particular for some Waterman), sometimes on the section (as used by the Aurora) or on the cap (typical of the Montblanc). Engravings on metal parts (clips, cap edge, head) are also classified as such, provided they refer to brand or model (as in the case of the Pelikan).
On the other hand, there is a tendency not to consider as real imprint (even if technically they are made in the same way and this distinction is not formally established in any way) the most elementary engravings that show only the number of the model, the gradation of the nib or the direction of rotation of a bottom.