Infusion
From Destinypedia, the Destiny wiki
Infusion is boosting the level of a favored Weapon or Armor piece, similar to Etheric Light. Infusing a stronger Light-leveled piece of gear into another less powerful one will have the level increased to that of Infused piece, but will not receive any of the perks, allowing for players to choose armor or weapons for aesthetic reasons and still have good "rolls", or perks they like or want.
Only Weapons and Armor from Year 2 and Year 3 are able to be Infused. This was a mechanic introduced at the beginning of Year 2 with the release of The Taken King.