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. They are the only energy weapon type that does not have a standard kinetic version (there is a stasis kinetic trace rifle, Ager's Scepter).
Currently, there are only eight trace rifles, (all except for Retraced Path and Hollow Denial are exotic): Coldheart, Prometheus Lens, Wavesplitter, Divinity, Ruinous Effigy, Ager's Scepter, Retraced Path, and Hollow Denial.
For a list of all trace rifles, see its category page.