An artificial heart is a device used to replace a heart.
In the 2320s, Jean-Luc Picard had to have an artificial heart replace his original after being stabbed by Nausicaans. (TNG episode: "Tapestry")
In 2365, Picard had to have it replaced after it malfunctioned. (TNG episode: "Samaritan Snare")
In 2374, Julian Bashir replaced Ishka's heart with an artificial replacement. (DS9 episode: "Profit and Lace")
In 2377, Kira Nerys' heart was damaged after being attacked by Taran'atar, forcing Bashir to replace it. (DS9 novels: Warpath, Fearful Symmetry)