A marker pen, fine liner, marking pen, felt-tip pen, felt pen, flow marker, sign pen, vivid, flomaster, texta, sketch pen, koki or simply marker is a pen which has its own ink source and a tip made of porous, pressed fibers such as felt. A marker pen consists of a container and a core of an absorbent material that holds the ink. The upper part of the marker contains the nib that was made in earlier times of a hard felt material, and a cap to prevent the marker from drying out.

A pen is a common writing instrument that applies ink to a surface, usually paper, for writing or drawing. Early pens such as reed pens, quill pens, dip pens and ruling pens held a small amount of ink on a nib or in a small void or cavity that had to be periodically recharged by dipping the tip of the pen into an inkwell. Today, such pens find only a small number of specialized uses, such as in illustration and calligraphy. Reed pens, quill pens and dip pens, which were used for writing, have been replaced by ballpoint pens, rollerball pens, fountain pens and felt or ceramic tip pens. Ruling pens, which were used for technical drawing and cartography, have been replaced by technical pens such as the Rapidograph. All of these modern pens contain internal ink reservoirs, such that they do not need to be dipped in ink while writing.