So I might be late to the party on this, but I just realized why the items in Destiny are "Engrams". Engram cells are neurons in the human brain that are responsible for memory. They essentially convert electrical impulses into semi-physical patterns and store them. When you them "remember" something, the same neurons are activated and the patterns are "rebuilt" into memories. It's why Rahool is always saying "Pattern is Engram, and Engram IS PATTERN!".