Ep ‧ i ‧ Pen /ˈepɪpen/ BrE AmE noun [countable] trademark
a medical instrument shaped like a large thick pen with a needle at one end, designed so that someone can ↑ inject a special drug into their body quickly and easily if they are having a severe ALLERGIC REACTION (=are extremely ill because of eating or touching a particular substance) or an ↑ asthma attack that might kill them.