Trace Rifles are a type of energy weapons introduced in Destiny 2. They fire a long beam of elemental energy, capable of dealing consistent damage while the beam is in contact with the target.
Currently, there are 12 trace rifles in the game, (all except for Retraced Path, Hollow Denial, and Path of Least Resistance are exotic): Coldheart, Prometheus Lens, Wavesplitter, Divinity, Ruinous Effigy, Ager's Scepter, Retraced Path, Hollow Denial, Path of Least Resistance, The Navigator, Appetence, and Microcosm.
For a list of all trace rifles, see its category page.