Pen juice is a term used to describe ink, but there are many different synonyms that can be used to describe this substance. One of the most common synonyms for pen juice is ink, which is the liquid substance used in pens and other writing instruments. Other synonyms include writing fluid, drawing fluid, and writing ink. Some people may refer to pen juice as pen ink, ballpoint ink, or even fountain pen ink depending on the specific type of pen being used. Regardless of what it's called, pen juice is an essential tool for writers, artists, and anyone else who needs to put something down on paper.