ミリペン
Written as「ミリペン」, this common abbreviation of "Millimetre Pen" refers to the instrument also known as a technical pen.  It has a hard tip and is known for making solid, consistent lines.  In this regard, it's easier to use than a [[nib pen (medium)|nib pen]], but there's a trade-off in terms of how much control you can have over your line.
millipen (medium)