Datto and others started to call certain hiding spots in Destiny cheese spots meaning that it safer but less exiting way to handle a fight. When Bungie released the first DLC called The Dark Below the word cheese started to morph into the description for exploiting a glitch in the system to change the mechanics of the game so that it is easier to handle. In the raid Crota’s End warlocks could use self-resurrection to pass the bridge without having a sword or to deal with sword bearer or using the host’s quitting of the Game to make Crota kneel “forever”.
Update: Later I say that DLC gear is not giving Radiant Shards / Energy when dismantled. That is not true for Raid Gear though but for any DLC gear (Lvl 36 for Armor) that is obtained otherwise.
In my book a cheese is the exploit of a glitch to change the game mechanics to make it easier and faster to beat. Read more